Squalene → Lanosterol

The first recognizable steroid ring system is lanosterol; it is formed first by the epoxidation of the double bond of squalene that was originally derived from a DMAPP through farnesyl pyrophosphate, and then by the cyclization of squalene epoxide. The enzyme that forms the epoxide uses NADPH to reduce molecular oxygen to make the epoxide. See Figure 1 .

                    Figure 1