Catalyzes the conversion of (3S)-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to mevalonic acid, the rate-limiting step in the synthesis of cholesterol and other isoprenoids, thus plays a critical role in cellular cholesterol homeostasis (PubMed:21357570, PubMed:2991281, PubMed:36745799, PubMed:6995544). HMGCR is the main target of statins, a class of cholesterol-lowering drugs (PubMed:11349148, PubMed:18540668, PubMed:36745799). {ECO:0000269|PubMed:11349148, ECO:0000269|PubMed:18540668, ECO:0000269|PubMed:21357570, ECO:0000269|PubMed:2991281, ECO:0000269|PubMed:36745799, ECO:0000269|PubMed:6995544}. This is the function of HMGCR (3-hydroxy-3-methylglutaryl-CoA reductase, ENSG00000113161).