For both psoriasis and PsA, a haplotypic effect at IL23R, with carriage of the common variants of both polymorphisms being associated with disease susceptibility, has been observed, suggesting that either both are acting as markers with an as yet ungenotyped variant with which they are both in linkage disequilibrium or that multiple susceptibility effects are present at the same locus. This evidence concerns the gene IL23R and psoriasis.