Catalyzes the condensation of 2 farnesyl pyrophosphate (FPP) moieties to form squalene (PubMed:9575210). 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 (By similarity). 2 (2E,6E)-farnesyl diphosphate + H(+) + NADPH = 2 diphosphate + NADP(+) + squalene 2 (2E,6E)-farnesyl diphosphate + H(+) + NADH = 2 diphosphate + NAD(+) + squalene H(+) + NADH + presqualene diphosphate = diphosphate + NAD(+) + squalene H(+) + NADPH + presqualene diphosphate = diphosphate + NADP(+) + squalene 2 (2E,6E)-farnesyl diphosphate = diphosphate + presqualene diphosphate Terpene metabolism; lanosterol biosynthesis; lanosterol from farnesyl diphosphate: step 1/3. Defects in Lss and Fdft1 are the cause of Shumiya cataract rat (SCR). This strain develop mature cataracts at around 11 weeks of age, exhibiting opacity from the perinuclear zone to the cortical intermediate layer. Cholesterol levels in cataractous lenses decreased to about 57% of normal. Cholesterol insufficiency may cause the deficient proliferation of lens epithelial cells in Shumiya cataract rats, resulting in the loss of homeostatic epithelial cell control of the underlying fiber cells and ultimately cataractogenesis. Belongs to the phytoene/squalene synthase family.