Catalyzes the condensation of 2 farnesyl pyrophosphate (FPP) moieties to form squalene. Proceeds in two distinct steps. In the first half-reaction, two molecules of FPP react to form the stable presqualene diphosphate intermediate (PSQPP), with concomitant release of a proton and a molecule of inorganic diphosphate. In the second half-reaction, PSQPP undergoes heterolysis, isomerization, and reduction with NADPH or NADH to form squalene. It is the first committed enzyme of the sterol biosynthesis pathway. 2 (2E,6E)-farnesyl diphosphate + H(+) + NADPH = 2 diphosphate + NADP(+) + squalene 2 (2E,6E)-farnesyl diphosphate + H(+) + NADH = 2 diphosphate + NAD(+) + squalene 2 (2E,6E)-farnesyl diphosphate = diphosphate + presqualene diphosphate H(+) + NADH + presqualene diphosphate = diphosphate + NAD(+) + squalene H(+) + NADPH + presqualene diphosphate = diphosphate + NADP(+) + squalene Terpene metabolism; lanosterol biosynthesis; lanosterol from farnesyl diphosphate: step 1/3. Widely expressed. The disease is caused by variants affecting the gene represented in this entry. Belongs to the phytoene/squalene synthase family.