The HMG-CoA reductases catalyze the conversion of HMG-CoA to mevalonate, which is the rate-limiting step in the synthesis of isoprenoids like cholesterol. Probably because of the critical role of this enzyme in cholesterol homeostasis, mammalian HMG-CoA reductase is heavily regulated at the transcriptional, translational, and post-translational levels.
Jiyao W et al.(2023). "The conserved domain database in 2023.",