Retinoids are polyisoprenoid lipids derived from retinol (vitamin A). They perform multiple regulatory functions in diverse cellular processes. Retinoids regulate cellular proliferation, differentiation , and death, and they have numerous clinical applications. Their diverse effects reflect, at least in part, the multiplicity of retinoid derivatives, the existence of two different classes of receptors that form heterodimers, and differences in their cis-acting regulatory sites on DNA . During development retinoids act as local mediators of cell-cell interaction. For instance, during the formation of motor neurons in the chick, one class of motor neurons generates a retinoid signal which regulates the number and type of neighboring motoneurons.