Our understanding of how vitamin D mediates biological responses has entered a new era. It is now clear that the bulk of the biological responses supported by vitamin D occur as a consequence of its metabolism to its daughter metabolite 1α,25‐dihydroxyvitamin D3(a steroid hormone). The fact that 1,25(OH)2D3receptors are ubiquitous in tissue distribution opens the possibility for unforeseen biological functions of the vitamin D endocrine system. For example, 1,25(OH)2D3serves as an immunoregulatory hormone and a differentiation hormone besides its classical role in mineral homeostasis. The avian 1,25(OH)2D3receptor has recently been cloned and shown to be a member of the nuclear transacting receptor family that includes estrogen, progesterone, glucocorticoid, thyroxine (T3), aldosterone, and retinoic acid receptors. We have compiled an extensive number of RNA polymerase II‐transcribed genes that are regulated by 1,25(OH)2D3. Classification of these genes on functional grounds identifies and formulates the several genetic circuits or biochemical systems in which 1,25(OH)2D3plays an essential regulatory role. These systems include genes that govern oncogene and lymphokine expression as well as those involved in mineral homeostasis, vitamin D metabolism, and regulation of a set of replication‐linked genes (c‐myc, c‐myb,and histone H4), which are critical for rapid cellular proliferation. An integrated analysis of the combinations of genetic circuits regulated by 1,25(OH)2D3suggests that they may be collectively tied to a DNA replication‐differentiation switch.— Minghetti, P. P.; Norman, A. W. 1,25(OH)2‐Vitamin D3receptors: gene regulation and genetic circuitry.FASEB J.2: 3043‐3053; 1988.