the primary objective of melanoma treatment is to specifically eradicate the tumor while minimizing damage to normal tissue . in order to accomplish this goal one group of proteins that exhibit selective expression in cancer includes a group of proteins whose expression is otherwise normally limited to germ cells . most of the research into germ cell proteins in cancer has focused on expression differences and immunogenic potential for vaccines . however there is an increasing effort to decipher the potential role germ cell proteins may play in oncogenesis . the first germ cell - specific antigen discovered was the melanoma antigen 1 ( mage - a1 ) in a patient with prolonged survival despite bulky lymph node disease . ongoing research revealed a family of mage antigens expressed in many tumor types , and while also expressed in the testis , the antigens did not appear to be expressed in most normal adult tissues . with the expansion of known germ cell proteins in cancer , the term cancer testis antigen ( cta ) was coined to refer to those proteins that are expressed primarily in the testis or placenta and cancers but not generally in normal adult tissues . nevertheless , cta expression can be found in normal tissues , such as pancreas , brain , and liver . the term cta has been limited to proteins not expressed in more than two nongerm cell normal tissues . however , clearly germ cell proteins may still play a role in cancer even if they are found to be more widely expressed . there are now over 100 families and 255 cancer testis entries compiled in a database by ludwig institute for cancer research . the common ctas researched in cancer include mage , gage , and ssx families as well as ny - eso1 and prame . the large number of these proteins and their expression in cancer suggests a potential link between germ cell pathways and tumor development . however , it is now clear that melanoma tumors are heterogeneous , and subpopulations of cells with stem - cell - like features are present . germ cells may be considered the ultimate stem cells as they have the capacity to give rise to entirely new individuals . some of the germ pathways are thought to have evolved from dna repair pathways and serve to specifically create genetic diversity in offspring . in single cell eukaryotes , such as yeast , these pathways are activated by stress [ 8 , 9 ] . it is possible that germ cell pathways play a critical role in allowing tumors to become more genomically diverse , survive under different metabolic conditions , and extend overall growth potential . despite recent advances in melanoma care , germ cell proteins may hold the key to new therapeutic approaches to prevent the development and evolution of cancer . this review will focus on differences in germ cell protein expression , role in diagnosis , prognosis , and therapy . further we will discuss critical germ cell pathways and consider the potential roles these pathways may play in malignant transformation . among malignancies , melanoma is one of the tumors with the highest frequency of cta expression . also included in this group are bladder , lung , ovarian , and hepatocellular cancers . the lowest levels of expression appear to be in lymphomas , renal and colon cancers . a correlation between the gene families expressed and cancer type has been identified . melanoma has been noted to express higher levels of magea , magec3 , spanx , and ldh than other cta gene families . the finding that germ cell proteins are not only differentially regulated across tumor types but they are also differentially regulated within tumor subpopulations suggests a potentially complex role in tumorigenesis . this is critically important in melanoma as the diagnosis of early melanoma versus an unusual benign mole can be quite challenging . a study by farmer et al . on the discordance of the histopathologic diagnosis of melanoma found that 38% of the samples reviewed had two or more discordant diagnoses . recurrent nevi , combined nevi , acral nevi , deep penetrating nevi and spitz nevi , may be overdiagnosed as melanoma while certain types of melanoma such as the nevoid , desmoplastic , spitzoid , and regressed lesions may be underdiagnosed . misdiagnosis of certain lesions may lead to unnecessary invasive treatment or a malignant melanoma going untreated . in order to improve patient care it is vital to develop more accurate diagnostic tests that will aid dermatopathologists and clinicians in the difficult task of classifying ambiguous lesions . the high expression of ctas in melanoma and lack of expression in normal skin make the presence of these germ cell proteins a potential diagnostic tool ( table 1 ) . primary melanomas and benign nevi have been analyzed based on immune detection of three antigens : mage - a1 ( ma454 ) , mage - a4 ( 57b ) , and ny - eso-1 ( es121 ) . when the cta panel was expanded to include three additional antigens including mage - c1 ( ct7 - 33 ) , mage - a3 ( m3h67 ) , and gage ( gage ) , 77% of the melanomas tested positive for at least one cta . therefore the sensitivity of potential diagnostic tests may be improved by increasing the number of antigens included in the array . other studies have found 100% specificity for mage-3 in melanoma nodal metastasis under optimal pcr conditions , but determined the need for a broader panel of ctas to increase the sensitivity . one study that used only anti - mage antibody 57b found the diagnostic potential of ctas was limited by their presence in several types of benign skin lesions . this reinforces the need for a wider range of antibodies in order to improve diagnostic abilities . the expression pattern revealed a statistically significant difference between normal skin , benign nevi , and melanoma . the prevalence of spanx in 80.9% of the melanomas tested makes it a useful target for diagnostic assays . normal skin did not show any presence of the antigen but benign nevi did display an intermediate level . this intermediate level was significantly lower than the expression in melanoma and significantly more than the expression in normal skin . additionally , mage-3 was found to be expressed in melanoma but not in normal melanocytes while prame was determined to be an adequate marker for differentiating between melanoma cells and benign nevi [ 15 , 18 ] . the expression of prame in 88% of primary cutaneous melanomas and lack of expression in normal nevi make it particularly useful [ 19 , 20 ] . thus germ cell protein / cta expression patterns may serve as a diagnostic tool to differentiate between malignant and benign skin lesions . in addition to potential diagnostic value , germ cell proteins may also have prognostic value . after melanoma diagnosis , clinicians determine the relative prognosis in order to anticipate further care needs . currently , prognostic information from the primary tumor is largely based on histopathologic criteria including breslow 's depth , ulceration , and mitotic rate . nevertheless , some lesions that should have had a good prognosis by these variables ultimately prove to be lethal and some with a poor prognosis never recur . a summary of ctas and their associated prognostic features is presented ( table 2 ) . according to the study by svobodov et al . expression of ctas has independent prognostic value for relapse - free survival ( rfs ) , with cta negative tumors ( mage - a1 , mage - a4 , and nyeso-1 ) having an rfs of 72 months compared to 45 months for positive tumors . this study found the cta prognostic value comparable to the current clinic - pathologic classifications . a study by mikhaylova et al . revealed a correlation between the level of expression of ctas and the differentiation of tumors with the more poorly differentiated cells exhibiting higher cta levels . further a study found a specific prognostic relationship between the stage of melanoma and antigen present . according to vourc'h - jourdain et al . , mage - a3 was significantly related to an increase in disease - free survival when expressed in stage iii melanoma patients . there is also potential prognostic value in determining the type of cta present in the tumor . according to velazquez et al . primary tumors that are ny - eso1 positive are thicker than those tumors negative for this antigen , while barrow et al . found ny - eso1 expression in tumors of 1.1 to 4  mm significantly higher than in thin tumors less than 1  mm . also , the ny - eso1 antigen is often more associated with metastatic disease [ 24 , 26 ] . combined thickness and metastasis properties result in a poor prognosis for ny - eso1 positive tumors . when comparing the presence of ny - eso1 to other antigens , like ctp11 , there is a positive correlation between ny - eso and more advanced disease while the ctp11 antigen is found in less advanced melanoma . certain antigens have been found exclusively in melanoma metastases and not in primary tumors , such as xage . another antigen , mage - a3 , was expressed at least partially in 90% of metastases , making it a positive indicator for metastatic melanoma . a study by curioni - fontecedro et al . found the expression of mage - c1 and mage - c2 in primary melanoma lesions to also be a significant predictor of lymph node metastasis . barrow et al . also found several correlations between mage - a1 and a4 and current prognostic criteria . mage - a1 showed a significant increase in expression in ulcerated and thicker tumors ; although mage - a4 displayed a similar trend , its correlation was not statistically significant . thus cta expression may have prognostic value in helping predict the potential aggressiveness of a tumor , potentially allowing clinicians to tailor therapeutic approaches accordingly . even without a complete understanding of germ cell protein function in cancers , there have been advances in germ cell protein - based therapies . however , until more is known about their function , the related therapies primarily revolve around germ cell protein immunogenicity . tumor cells are generally considered antigenic but not immunogenic , allowing them to evade host immune defenses . the limited expression of cancer testis antigens has made them a major target for immune - based therapies because their restricted expression should cause minimal side effects . melanoma - specific vaccines have been created to specific germ cell proteins in an effort to drive the patient 's own immune system to attack the cancer cells . there are several current trials to test the efficacy of such vaccines in melanoma patients , particularly mage and ny - eso1 antigens . in addition to vaccines it may be possible to make the tumors more antigenic by driving the expression of ctas . there is evidence that regulation of cancer testis genes , like mage , is dependent on dna methylation and histone acetylation . this makes epigenetic regulation by way of histone deacetylase inhibitors and dna demethylating agents a possible treatment [ 32 , 33 ] . kit tyrosine kinase may also be important in the epigenetic control of mage and potentially a target for future therapies . found t cell receptor modification of transduced t cells , specific for ny - eso1 , to be an effective therapeutic approach for melanoma . finally , combination therapies utilizing several of these approaches may have synergistic effect in treating melanoma . in addition to the role of ctas as direct immunologic targets , germ cell protein expression may also provide clinicians with insight into tumor sensitivity to certain drugs . cell lines which express at least one of three mage genes are more susceptible to tumor necrosis factor - mediated cytotoxicity , while the over - expression of genes like mage - a2 and mage - a6 may signal resistance to chemotherapy . the differences in susceptibility show the potential for tailoring therapies based on the expression of germ cell genes . thus germ cell proteins may not only serve as direct immunologic targets but they may also direct the pharmacologic therapeutic approach . while the presence of germ cell proteins in malignancies is well documented , the reason why they are expressed is not clearly understood . theories on germ cell protein expression can be grouped into three general categories including ( 1 ) accidental / nonfunctional , ( 2 ) accidental / functional , and ( 3 ) programmed / functional . the first is an accidental activation of germ cell - specific genes , which do not provide any functional benefit to the cancer cell . this scenario would include processes like widespread epigenetic regulation that would turn on germ cell genes secondarily but without any benefit to the cell . the most noted epigenetic controls in germ cell gene expression appear to be dna methylation and posttranslational histone modifications . the second category consists of the accidental activation of germ cell genes , but with a functional benefit to the cancer cells . by providing a benefit , the cancer cells that express these germ cell genes may survive and thrive better than those without expression thereby expanding the population expressing beneficial germ cell proteins . lastly , the expression of germ cell proteins may be programmed into the cell to be activated under certain conditions . primitive organisms , like yeast , tend to increase genetic recombination when they experience such stresses in order to create new phenotypes that may be better suited for the environment . a recent study by forche et al . found a correlation between loss of heterozygosity in candida albicans and the degree of stress the yeast was exposed to , suggesting increased levels of gene rearrangement . due to overgrowth of the local blood supply , a fraction of cancer cells would be expected to be hypoxic and possibly nutrient deprived . an evolutionary conserved programmed response to the lack of oxygen and nutrients could lead to the activation of germ cell proteins , causing genetic recombination , and genomic instability , similar to that seen in yeast , allowing cancer cells to adapt to the environment and become more suited to thrive in adverse conditions . in all three scenarios it would be expected that there would be differential expression of germ cell proteins in the tumor mass as the cancer progresses . specifically , heterogeneity has been noted in the expression of ssx , gage , and ny - eso1 [ 4547 ] . evidence has been found for an increase in germ cell protein expression with tumor progression . a study by barrow et al . found that both antigens mage - a1 and mage - a4 showed an increase in expression throughout disease progression . these patterns suggest that germ cell protein expression may be playing a role in tumor progression . the increase in expression may suggest that the germ cell phenotype confers adaptations more suitable for survival in adverse conditions . if these germ cell - specific genes are being turned on as a programmed response to stressors , their function is likely significant to survival of the cancer cells . therefore , disrupting this function may decrease the ability of cancer cells to adapt and thrive under stressful conditions . there is increasing evidence that germ cell pathways can be activated and may play a role in tumorigenesis . hypoxic stress has been noted to induce germ cell protein expression in rat kidney fibroblasts , suggesting that hypoxia by itself may be enough to turn on some of these pathways . the germ cell regulatory protein , plu1 ( jarid1b ) , has been shown to mark a subpopulation of melanoma tumor cells required for continuous tumor growth . further an association between germ cell gene expression and brain tumors in drosophila has recently been identified . knock - down experiments of several of the germ cell proteins revealed that they played a critical role in tumor growth . together these studies support the idea that germ cell pathways may be activated due to stress or other means and that these proteins then functionally benefit the malignant state . although it is possible that germ cell protein expression is accidental / nonfunctional in cancer , given the association of upregulation with hypoxia and role in brain tumor development , it is more likely that expression of these pathways is programmed and functional in tumor development . although there have been significant advances in unraveling the pathways involved in tumorigenesis , the role of germ cell proteins in this process remains to be fully understood . a recent review by fratta et al .   covered many of the known molecular functions of ctas ; these findings will also be briefly addressed below in the apoptosis / transcriptional regulation sections . in addition to these findings , germ cell proteins affect other major potential pathways such as metabolism , meiosis , and telomere extension that are likely to play critical roles ( table 3 ) . genes of the class i mage family have been found to be associated with the p53 corepressor , kap1 . the suppression of mage by sirna and small compounds has been shown to inhibit tumor growth and induce apoptosis [ 51 , 52 ] . the prame gene was found to repress retinoic acid signaling , a common proliferation inhibitor and apoptosis inducer . by interfering with retinoic acid receptors , thus expression of these germ cell proteins may help the cancer cells escape programmed cell death . mage - a1 was found to inhibit transcription by interacting with the transcriptional regulator , skip , and recruiting histone deactlyase 1 ( hdac1 ) . skip interacts with the notch pathway , which controls cell differentiation during embryonic and adult life . germ cells express a unique set of metabolic enzymes that allow them to utilize certain substrates more effectively . spermatocytes are able to utilize lactate , pyruvate , and glucose while spermatids are only able to use lactate . there are a number of glutamate transporters that are preferentially expressed in certain stages of spermatogenesis and allow for increased or decreased utilization of glucose . there are also specific glycolytic enzymes expressed only in spermatogenic cells , including hexokinase ( hk ) , phosphoglycerate kinase-2 ( pgk2 ) , and glyceraldehyde 3-phosphate dehydrogenase ( gapd ) . increased glucose utilization is an important finding in many cancers and has been referred to as the warburg effect . in this phenomenon cancer cells utilize the glycolytic pathway in a fermentative manner , resulting in an increase in lactate . even in the presence of oxygen the cancer cells rely more on fermentation than respiration . expression of germ cell enzymes may contribute to the preferential use of glucose by cancer cells . the ability of germ cells to utilize lactate is also dependent on a testis - specific enzyme , lactate dehydrogenase c or ldhc . expression of this enzyme is found in numerous human cancers , allowing the cancers to use lactate as a substrate for atp production . this is beneficial to the cancer cells in light of the warburg effect , which results in a buildup of lactate within the cell . melanoma cell lines , when compared to normal melanocytes , rely more heavily on the warburg effect . while under hypoxic conditions melanocytes and melanoma cells both showed signs of glucose fermentation to lactate , only the melanoma cells were able to use the tricarboxylic acid cycle to produce fatty acids from glutamine and lactate from glucose . the increase of lactate production and utilization in melanoma cells is accompanied by an upregulation of the germ cell protein ldhc . the change in metabolism seen in cancer cells is potentially attributable to this expression of alternate enzymes that are normally expressed in germ cells . tumors may exploit these proteins to adapt to changes in substrate concentrations that accompany abnormal cell proliferation . this display of metabolic adaptations supports the theory of programmed expression of germ cell proteins because by expanding a cell 's useable substrates for energy , the cell has an increased chance of survival . cell division requires cell cycling through mitosis ; however germ cells also have the capacity to undergo meiosis . unlike mitosis , in the first meiotic division the sister chromatids remain attached to each other and recombination occurs across homologous chromosome arms . the expression of meiosis proteins in cells attempting to undergo mitosis ( termed meiomitosis ) could cause genomic instability . these include spo11 a protein that creates double - strand dna breaks : scp1 , a protein involved in the pairing of homologous chromosomes   and hormad1 which may play a regulatory role in meiotic synapses . there are also several other meiotic proteins that may be present in melanomas such as rec8 , a meiosis - specific cohesion ( figure 1 ) . during meiosis i rec8 is maintained at the centromeres and functions to bind the sister chromatids together . in meiosis the expression of rec8 during mitotic division could lead to failure of nuclear division , abnormal chromosomal segregation , and aneuploidy . thus meiosis proteins have been noted to be expressed in melanoma and could function to cause genomic instability . since telomeres shorten with every round of division , cancer cells must express a mechanism to maintain telomere length in order to protect the genetic material . in order to accomplish this task , over 90% of cancers reactivate telomerase , suggesting it is vital to their survival . one of the highest rates of normal telomerase expression is in the testis just prior to meiosis i in the primary spermatocyte . the levels of telomerase expression have been noted to be significantly higher in melanoma than in acquired and dysplastic nevi . in contrast to cell lines that fail to continue to grow in culture , immortal melanoma lines maintain telomerase expression . it is possible that the pathways leading to the activation of telomerase also activate other germ cell proteins . in summary , the expression of germ cell proteins in melanoma has the capacity to prevent cell death , alter transcription , improve energy options , promote evolution of the genome , and provide infinite growth potential . the expression of these proteins in melanoma has been demonstrated to have both diagnostic and prognostic value . it is likely that they contribute to altered metabolism , immortalization , and genomic instability in melanoma and other cancers . the high level of expression of germ cell proteins within melanoma makes the disease an ideal model for dissecting their role in tumorgenesis . these efforts may provide insights into microenvironmental processes that differentially regulate the germ cell genes which in turn likely serve to drive tumorgenesis . ultimately research unraveling these pathways may allow for the development of new therapeutic targets to control or potentially eradicate tumor cell growth .