Presqualene pyrophosphate is an intermediate in cholesterol and steroid biosynthesis (Figure 19.21). It is made in the membrane of the endoplasmic reticulum by joining two farnesyl pyrophosphates in the reaction that follows:

2 Farnesyl Pyrophosphate <=> Presqualene Pyrophosphate

Presqualene pyrophosphate is subsequently converted to squalene, also in the membrane of the endoplasmic reticulum (Figure 19.21).