In mammals, retinol is the precursor for retinoids, which affect various aspects of morphogenesis and development. However, B lymphocytes, although retinol-dependent, do not use retinoic acid as mediator. Retinol is metabolized by B lymphocytes and other cell lines to optically active 14-hydroxy-4,14-retro-retinol; it is this compound that mediates the growth control. Thus another second messenger molecule, in addition to retinoic acid and retinal, is derived from retinol.