Squalene synthase; part of the third module of ergosterol biosynthesis pathway that includes the late steps of the pathway (By similarity). The third module or late pathway involves the ergosterol synthesis itself through consecutive reactions that mainly occur in the endoplasmic reticulum (ER) membrane (By similarity). Firstly, the squalene synthase SQS catalyzes the condensation of 2 farnesyl pyrophosphate moieties to form squalene, which is the precursor of all steroids (PubMed:18051320). Secondly, the squalene epoxidase catalyzes the stereospecific oxidation of squalene to (S)-2,3-epoxysqualene, which is considered to be a rate-limiting enzyme in steroid biosynthesis (By similarity). Then, the lanosterol synthase LS catalyzes the cyclization of (S)-2,3 oxidosqualene to lanosterol, a reaction that forms the sterol core (PubMed:20460708). In the next steps, lanosterol is transformed to ergosterol via a complex process involving various demethylation, reduction and desaturation reactions (By similarity). Lanosterol is also an intermediate in the biosynthesis of triterpenes such as ganoderic acids (GA), a group of highly oxygenated lanostane-type triterpenoids which are well recognized as a main group of unique bioactive compounds in the medicinal mushroom Ganoderma lucidum (Probable). 2 (2E,6E)-farnesyl diphosphate + H(+) + NADPH = 2 diphosphate + NADP(+) + squalene 2 (2E,6E)-farnesyl diphosphate + H(+) + NADH = 2 diphosphate + NAD(+) + squalene Terpene metabolism; lanosterol biosynthesis; lanosterol from farnesyl diphosphate: step 1/3. Expression is relatively low in mycelia and reaches the highest level in the primordia. Expression is induced in ganoderic acid (GA) producing conditions. Belongs to the phytoene/squalene synthase family.