Psoriasis (OMIM#177900) is a common polygenic skin disorder affecting approximately 2% of the northern European population and 0.1% of the Han Chinese. Psoriasis patients suffer from chronic skin inflammation, manifested by erythematous scaly lesions. PSORS1-PSORS9 have been confirmed as psoriasis susceptibility loci in independent genetic studies on predominantly Caucasian populations, with psoriasis susceptibility loci (PSORS1, PSORS9) and additional loci at 9q33-34 and 2p22.3-11.2 reported in Han Chinese patients. In this study, we show the association of PSORS4 with psoriasis in Singaporean Chinese. Dense genotyping of single-nucleotide polymorphism-tagging candidate genes within the epidermal differentiation complex revealed significant association in the proximity of the involucrin gene (IVL); the strongest association was seen in early-onset psoriasis patients (P=0.0014). A follow-up genome-wide association screen localized the psoriasis susceptibility region to ~360 kb along chromosome 1 in the vicinity of IVL, small proline-rich region (SPRR) and proline-rich region 9 (PRR9) genes. The study of interactions between the causative variant(s) in this locus will provide insights into a possible role for epidermal barrier formation in the pathogenesis of psoriasis.