Cholecalciferol (vitamin D3), an example of a 9,10-secosteroid. The hydroxyl group (HO-) is in position C3 of the parent steroid A-ring. The triene substructure attached to the ring bearing the hydroxyl group is a result of the ring scission (cleavage) giving rise to this secosteroid.
The parent steroid skeleton. The B-ring of the parent steroid is broken between C9 and C10 to yield the vitamins D.
A secosteroid (sec·o·ster·oid, sek'ō-stēr'oyd) is a type of steroid with a "broken" ring. The word secosteroid derives from the verb Latin: secare meaning "to cut",:241 and Latin: stere of steroid, meaning "solid, three-dimensional".:129 Secosteroids are alternatively described as a subclass of steroids or derived from steroids.
Types or subclasses of secosteroids are defined by the carbon atoms of the parent steroid skeleton where the ring cleavage has taken place. For example, 9,10-secosteroids derived from cleavage of the bond between carbon atoms C9 and C10 of the steroid B-ring (similarly 5,6-secosteroids, 13,14-steroids, etc.).