Topical retinoids: in vivo predictive assays. 
Vitamin A is important for epithelial cell proliferation and differentiation and therefore has been used as a therapy for a variety of skin diseases.  Synthetic vitamin A derivatives, called retinoids, have been used to treat a number of clinical skin conditions, including cystic acne and various forms of ichthyosis, Darier's disease, and psoriasis.  Because of the toxicity of existing retinoids, newer vitamin A derivatives have been synthesized and must be evaluated in preclinical assays.  Retinoids have profoundly differing biologic properties that can be evaluated selectively by studying their efficacy in different in vivo animal assays.  This review will discuss some of the assays that may be useful predictors of retinoid efficacy.