Monoterpene synthase that catalyzes the formation of myrcene. Can use geranyl diphosphate as substrate, but not farnesyl diphosphate or geranylgeranyl diphosphate. (2E)-geranyl diphosphate = beta-myrcene + diphosphate Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed in trichomes. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsb subfamily.