RESEARCH directed at defining the molecular mode of action of vitamin D is currently at its apex. There is now evidence implicating the essential involvement of vitamin D metabolites in a host of cellular processes, including calcium homeostasis, immunology, cell differentiation, and regulation of gene transcription. Further, there is evidence that the hormonally active form of vitamin D, 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃], may generate biological responses via both regulation of gene transcription as well as via nongenomic pathways, some of which involve opening of voltage-gated Ca²⁺ channels. In addition, there are many examples of pathological disruption of the normal state in which a drug form of a vitamin D metabolite is proposed to be a (potentially) useful form of treatment, e.g. renal osteodystrophy, psoriasis, leukemia, breast cancer, and osteoporosis.

The importance of the molecule vitamin D in the biological systems of higher animals has been recognized since its discovery by Mellanby in 1920 (1). It was in the interval of 1920–1930 that vitamin D officially became classified as a “vitamin” that was essential for the normal development of the skeleton and maintenance of Ca²⁺ homeostasis. The chemical structure of vitamin D was not determined until 1932 (2), and it was only then that it was apparent that this important nutritional substance was in reality a steroid, more specifically, a secosteroid, indicating that one of the rings of the cyclopentanoperhydrophenanthrene ring structure (the 9–10 carbon-carbon bond of ring B) was broken (see Section II).