An interesting potential type of biomarker is the presence of single nucleotide polymorphisms (SNPs). Such polymorphisms are alterations in (i) genes which may be associated with genetic susceptibility to particular diseases or (ii) genes which are important in molecular pathways involved in disease and/or treatment response. Many respective genes in these pathways have been implicated by genome-wide association studies. For example, in terms of psoriasis, SNPs have been identified in cellular methotrexate efflux transporters which may influence both treatment response and the risk of methotrexate toxicity. On the other hand, variations in the genes important to the intracellular metabolism of methotrexate did not predict treatment response to methotrexate. Interestingly, variations in the IL17A and IL17A receptor gene are reportedly not associated with psoriatic arthritis susceptibility, although such variations remain to be systematically examined specifically in patients with psoriasis.