A chemical group composed of multiples of the branched, unsaturated, 5-carbon isoprene unit, e.g. geranyl (10 carbons), farnesyl (15 carbons), geranylgeranyl (20 carbons). It is sometimes attached to the thiol group of an O-methylated C-terminal cysteine residue that forms part of a CAAX box, where C is the cysteine, A is any amino acid with an aliphatic side chain and X is any amino acid; a prenylpyrophosphate donates the prenyl group to the -CAAX sequence, AAX is proteolytically cleaved and the exposed carboxy group of the derivatized cysteine residue is enzymically methylated by S-adenosylmethionine. Prenylated proteins serve in the control of cellular function. Vitamins A and K, and dolichol pyrophosphate, carry reduced prenyl groups.