Screening for TSD is carried out with two possible objectives: * Carrier testing seeks to detect whether an individual unaffected by the<unk>  is carrying one copy of a mutation. Individuals seeking carrier screening are couples from at-risk populations who are seeking to start a family. Individuals and couples who seek carrier screening are aware of test results or genetic<unk>  in ancestors or living family members. * Prenatal testing seeks to determine whether the fetus has inherited two defective copies, one from each parent. In prenatal testing, there is generally greater information about family history and the mutations are often known precisely. Prenatal testing for TSD is usually undertaken when both parents cannot be ruled out as possible carriers. Prenatal testing can be performed by assay of HEX A enzyme activity in fetal cells obtained by chorionic villus sampling or amniocentesis. If an actual mutation has been identified in both parents, then more precise mutational analysis techniques using PCR are available. Two technical approaches to testing for<unk><unk><unk>  mutations are available. The enzyme assay approach tests phenotype at the molecular level with the levels of enzyme activity, while the mutation analysis approach tests the genotype directly, seeking known genetic markers. As with all biomedical tests, both approaches can produce false positives and false negative results. The two methods are used in tandem because an enzyme assay can detect all mutations with some inconclusive results, while mutation analysis can give definite results, but only for known mutations. Family history can be used to select a more effective testing protocol. Both carrier and prenatal testing using enzyme assay became available in the 1970s. Mutation analysis was added to testing protocols gradually after 1990 as the costs of PCR techniques declined. Over time, as knowledge of the mutation base has increased, mutation analysis has played an increasingly significant role. Due to the sensitive nature of testing in parental decisions, beginning in the 1970s wrongful life suits began to emerge. This was most common among parents who were not previously aware of their risk factors and felt that their doctors had the responsibility to inform them of their genetic risks. Enzyme assay techniques Enzyme assay techniques detect individuals with lower levels of hexosaminidase A. Development of a serum enzyme assay test made it feasible to conduct large scale screening for<unk><unk><unk>  in targeted at-risk populations such as Ashkenazi Jews. Developed in the late 1960s and then automated during the 1970s, the serum test was a first in medical genetics. It produced few false positives among Ashkenazi Jews, the first group targeted for screening. In enzyme assay, success with one targeted population cannot always be generalized to other populations, because the mutation base is diverse. Different mutations have different effects on enzyme assay results. Polymorphisms are neutral, while others affect the phenotype without causing<unk> . Enzyme assay was particularly effective among Ashkenazi Jews because fewer pseudodeficiency alleles are found in this population, as compared with the general population. Because serum can be drawn at low cost and without an invasive procedure, it is the preferred tissue for enzyme assay testing. Whole blood is normally drawn, but the enzyme assay measures activity in leukocytes, white blood cells that represent only a small fraction of whole blood. Serum testing gives inconclusive results in about 10% of cases when used to screen individuals from the general population. Serum testing also cannot be used to test pregnant women or women using hormonal birth control pills. To address these deficiencies, other techniques using enzyme assay have been developed. Mutation analysis techniques Although early testing for human mutations was often conducted by extracting DNA from larger tissue samples, modern testing in human subjects generally employs polymerase chain reaction because small tissue samples can be obtained by minimally invasive techniques, and at very low cost. PCR techniques amplify a sample of DNA and then test genetic markers to identify actual mutations. Current PCR testing methods screen a panel of the most common mutations, although this leaves open a small probability of both false positive and false negative results. PCR testing is more effective when the ancestry of both parents is known, allowing for proper selection of genetic markers. Genetic counselors, working with couples that plan to conceive a child, assess risk factors based on ancestry to determine which testing methods are appropriate. Mutation analysis techniques have declined rapidly in cost since the 1980s, a development that has run parallel with advances in computation and information processing technology. At the same time, knowledge of mutations has increased, allowing researchers and practitioners to interpret mutation data. It will soon be cost efficient to sequence and analyze the whole HEXA gene in at-risk specimens. Biotechnology offers the prospect that in the future, all individuals, even those without known risk factors, will be able to afford a full genome sequence report (see personal genomics). Such screening would identify novel as well as known mutations. As the cost of direct mutation analysis declines, medical genetics will confront the fact that full sequencing of the genome identifies polymorphisms that are neutral or harmless. This prospect will create uncertainty for couples using full genome sequencing methods. Czech medical geneticist Eva Machácková writes: "In some cases it is difficult to distinguish if the detected sequence variant is a causal mutation or a neutral (polymorphic) variation without effect on phenotype. The interpretation of rare sequence variants of unknown significance detected in<unk> -causing genes becomes an increasingly important problem." In the Jewish population Screening for<unk><unk><unk>  carriers was one of the first great successes of the emerging field of genetic counseling and diagnosis. Proactive testing has been quite effective in eliminating<unk><unk><unk>  occurrence among Ashkenazi Jews, both in Israel and in the diaspora. In 2000, Michael Kaback reported that in the United States and Canada, the incidence of TSD in the Jewish population had declined by more than 90% since the advent of genetic screening. On January 18, 2005, the Israeli English language daily Haaretz reported that as a "Jewish<unk> "<unk><unk><unk>  had almost been eradicated. Of the 10 babies born with<unk><unk><unk>  in North America in 2003, none had been born to Jewish families. In Israel, only one child was born with<unk><unk><unk>  in 2003, and preliminary results from early 2005 indicated that none were born with the<unk>  in 2004.
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