Retinoic acid(RA),which can be synthesized in vivo with vitamin A uptaken from food in vertebrates,takes part in the growth and differentiation of many types of cells.especially in the differentiation of epidermal cells.RA has been used in the treatment of skin disorders for a long time.but possible side effects have dramatically limited its clinical application.For example,difluse hair loss is a common side effect,and the nature of it is silll obscure.The immunolocalization of proteins related to retinoic acid synthesis and signaling during hair cycle may provide some clues to the investigation into the mechanisms underlying retinoid-induced effluvium.