Psoriasis is a debilitating, chronic inflammatory skin disorder characterized by scaly skin patches caused by infiltration of inflammatory cells into the dermis and epidermis, with secondary epidermal cell (keratinocyte) hyperproliferation. Psoriasis is a complex disease and a combination of genetic and environmental factors are likely to be causative. T-cell-mediated immune process, including cytokines (eg. IFN-γ) and chemokines, play an essential role in psoriasis. Susceptibility genes for psoriasis map to PSORS1 in the HLA region at chromosome 6p21, PSORS2 in the chromosome 17q24-q25, and others.