Extract: Psoriasis, a common inflammatory skin disorder, has recently moved into the limelight of both basic and clinical research. On the one hand, research into its pathogenesis has furthered our general understanding of T cell-mediated autoimmune disorders, and, on the other hand, psoriasis is used increasingly as a primary target disorder for novel therapies that are pathogenesis-oriented. Given that psoriasis affects approximately 2% of the population, it is a truly common skin disease. It is, therefore, somewhat surprising that the first description of psoriasis as a distinct entity dates back only to the year 1841. Geographic and ethnic factors appear to have a significant influence on the prevalence of psoriasis: ranges from 0% in the population of the Pacific islands of Samoa to 12% in the Arctic Kasach'ye have been reported. Ethnic influence is particularly evident when looking at the prevalence in African Americans, which is less than half that of the United States in general. Numerous family studies have provided compelling evidence for a genetic predisposition to develop psoriasis, although the inheritance pattern is still unclear. Genome-wide linkage studies have identified several putative psoriasis susceptibility loci, one of which located in the MHC (major histocompatibility complex, a cluster/locus of genes involved in the immune response of rejection) region on chromosome 6 was found to be present in several populations. This locus, termed "PSORiasis Susceptibility 1" (PSORS1), can thus be considered the major susceptibility locus and is associated with up to 50% of psoriasis cases.