Steroid skeleton. Note how the "B" ring is broken in Vitamin D.
A secosteroid is a molecule similar to a steroid but with a "broken" ring.
Secosteroids are very similar in structure to steroids except that two of the B-ring carbon atoms (C9 and 10) of the typical four steroid rings are not joined, whereas in steroids they are.
In humans, the most important secosteroid is Vitamin D.
