Squalene synthase; part of the gene cluster that mediates the biosynthesis of squalestatin S1 (SQS1, also known as zaragozic acid A), a heavily oxidized fungal polyketide that offers potent cholesterol lowering activity by targeting squalene synthase (SS) (PubMed:28605916). Catalyzes the condensation of 2 two farnesyl pyrophosphate moieties to form squalene (By similarity). The presence of a gene encoding a squalene synthase supports the identification of the cluster as being responsible for SQS1 production and suggests a likely mechanism for self-resistance (Probable). 2 (2E,6E)-farnesyl diphosphate + H(+) + NADH = 2 diphosphate + NAD(+) + squalene 2 (2E,6E)-farnesyl diphosphate + H(+) + NADPH = 2 diphosphate + NADP(+) + squalene Terpene metabolism; lanosterol biosynthesis; lanosterol from farnesyl diphosphate: step 1/3. Belongs to the phytoene/squalene synthase family.