Farnesyl diphosphate farnesyltransferase (FDFT; squalene synthase) catalyzes the reductive dimerization of two farnesyl diphosphate (FPP) molecules to form squalene. This happens in two distinct steps. The first step of dimerization forms presqualene diphosphate (Pandit et al. 2000).
Harwood HJ, Jr,
farnesyl-diphosphate farnesyltransferase activity of FDFT:Mg2+ [endoplasmic reticulum membrane]