K5.Stat3C transgenic mice, in which Stat3 is constitutively active in keratinocytes, develop psoriasis-like lesions following wounding stimuli or topical treatment with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), which strongly suggests that Stat3 activation is required for the development of psoriasis. This evidence concerns the gene STAT3 and neoplasm.