Squalene synthase is a member of the Isoprenoid Synthase Type I superfamily. These enzymes catalyse the formation of squalene by the reductive dimerisation of two farnesyl diphosphate molecules in a two-step reaction (2 farnesyl diphosphate --> presqualene diphosphate; presqualene diphosphate + NAD(P)H --> squalene). In the human enzyme, the first half reaction occurs at one end of a large channel (surrounded by five alpha helices runs through the centre of the protein). Then the intermediate is thought to move into an enclosed pocket where the second half reaction occurs. This reaction occurs at the final branch point of the isoprenoid biosynthesis pathway, and is the first committed step in cholesterol biosynthesis.