Blnstitut de Genetique et de Biologie Moleculaire et

Differential control of gene expression has become a cen-tral theme in modern molecular biology. The lipophilic hor-mones, because they are able both to diffuse from asource and permeate to a target, are ideal candidates to serve as regulators of this process. These hormones, including the steroids, retinoids, thyroid hormones, and vitamin DS, are potent regulators of development, cell differentiation, and organ physiology (Figure 1). Historically, they were isolated in the early part of this century based on their abilities to affect development, differentiation, metamor-phosis, and physiology. In addition, many of these hor-mones were associated with known human diseases, and by the middle of the century glucocorticoids had gained popularity as a therapeutic agent. While the mediators of the action of these hormones remained elusive, develop