Moreover, among them, IL-17, which is mainly produced by γδ T cells, CD4+ helper T cells (Th17 cells), and CD8+ cytotoxic T cells (Tc17 cells), seems to be most strongly implicated in psoriasis; thus, T cells, especially helper T (Th) cells, have become a hot topic in psoriasis pathogenesis. This evidence concerns the gene CD4 and psoriasis.