An enzyme that catalyzes the synthesis of hydroxymethylglutaryl-CoA from acetyl-CoA and acetoacetyl-CoA. This is a key enzyme in steroid biosynthesis. This enzyme was formerly listed as EC 4.1.3.5.

Synonyms: HMG CoA Synthase, A synthase, 3-hydroxy-3-methylglutaryl-coenzyme, HMG-CoA synthase, synthase, 3-hydroxy-3-methylglutaryl-coenzyme A, Synthase, Hydroxymethylglutaryl-CoA, synthase, HMG-CoA, 3-hydroxy-3-methylglutaryl-coenzyme A synthase, 3 hydroxy 3 methylglutaryl coenzyme A synthase, CoA Synthase, HMG, Hydroxymethylglutaryl CoA Synthase, (S)-3-Hydroxy-3-methylglutaryl-CoA acetoacetyl-CoA-lyase (CoA-acetylating), EC 2.3.3.10, Hydroxymethylglutaryl-CoA Synthase, Synthase, HMG CoA