Catalyzes the conversion of malonyl-CoA to acetyl-CoA. In the fatty acid biosynthesis MCD selectively removes malonyl-CoA and thus assures that methyl-malonyl-CoA is the only chain elongating substrate for fatty acid synthase and that fatty acids with multiple methyl side chains are produced. H(+) + malonyl-CoA = acetyl-CoA + CO2 Metabolic intermediate biosynthesis; acetyl-CoA biosynthesis; acetyl-CoA from malonyl-CoA: step 1/1. Mitochondrial in liver. Cytoplasmic in uropygial gland.