Psoriasis is a chronic immune-mediated disease that is characterized by the hyperproliferation and abnormal differentiation of keratinocytes and by inflammation. The epidermal changes associated with psoriasis may be due to the infiltration of inflammatory T lymphocytes and the release of cytokines in response to antigenic stimulation. Tazarotene is a retinoic acid receptor-specific retinoid with demonstrated efficacy in the topical treatment of psoriasis. Tazarotene downregulates markers of keratinocyte differentiation, keratinocyte proliferation, and inflammation. The drug also upregulates three novel genes TIG-1 (tazarotene-induced gene-1), TIG-2, and TIG-3, which may mediate an antiproliferative effect. The effect of tazarotene on these markers is probably a direct effect on gene expression rather than an indirect effect associated with disease improvement. (J Am Acad Dermatol 1997; 37:S18–S24.)