Secosteroids are steroid compounds with an open Bring commonly found in fungi (vitamin D 2), plants, invertebrates and vertebrates (vitamin D 3). Evolutionary, vitamin D 3 is a very old molecule, [1] which is created in a non-enzymatic reaction when the direct cholesterol precursor 7-dehydrocholesterol is exposed to UV-B (290-320 nm), [2,3] that is, cholesterol synthesizing species use(d) vitamin D 3 for UV-B scavenging [4] (Figure 1). This implies that also humans can synthesize vitamin D 3 in their skin. Through the action of a variety of cytochrome P450 (CYP) enzymes numerous vitamin D metabolites are created, [5] of which 25-hydroxyvitamin D 3 (25(OH)D 3) is the most stable (serum half-life some 3 weeks [6]) and abundant (a vitamin D sufficient person should have 25(OH)D 3 serum concentration of at least 75 nM [7]). The starting point of what is called today vitamin D endocrinology was some 550 million years ago, when in a boneless fish a receptor evolved that bound with high affinity the metabolite 1α,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3). [8] This high affinity