The predominant sterol in fungal cell membranes is ergosterol. This enzyme participates in the sterol biosynthesis pathway that leads from lanosterol to ergosterol. Stanols are saturated sterols, having no double bonds in the sterol ring structure. Experimentally, resistance usually arises as a result of mutations in the sterol biosynthesis pathway. Lanolin primarily consists of sterol esters instead. Cholesterol is a sterol, a steroid-like lipid made by animals, including humans. It is a sterol (or modified steroid), a type of lipid. Transcription of the reductase gene is enhanced by the "sterol regulatory element binding protein" (SREBP). In animal cells, membrane fluidity is modulated by the inclusion of the sterol cholesterol. Cholesterol is converted mainly into coprostanol, a nonabsorbable sterol that is excreted in the feces. Cholesterol is a tetracyclic alcohol and a type of sterol. In yeast, acyl-CoA&colon;sterol acyltransferase (ASAT) is functionally equivalent to ACAT. Diagnosis is made by measuring serum plant sterol concentrations. Reactivity with the membrane is also sterol concentration dependent. The best known sterol is cholesterol, which is found in humans.