S100A1 and psoriasis: Furthermore, the expression of several genes relevant in psoriasis pathogenesis were altered, these including cytokeratins 6, 16, and, 17, and genes of the epidermal differentiation complex region on the PSORS4 locus (1q21), such as S100 calcium binding protein A1 (S100A) and small proline-rich protein (SPRR) [11].