Diesters of phorbol, particularly 12-O-tetradecanoylphorbol 13-acetate, also known as phorbol myristate acetate, have been identified as the potent, highly lipophilic tumor-promoting components in croton oil. They are potent promoters of skin tumors in vivo and have been shown to transform cultured fibroblasts and embryonic cells previously exposed to polycyclic aromatic hydrocarbon carcinogens in vitro. The structure of phorbol, the parent alcohol is shown below. Phorbol esters also have important effects on leukocytes and on the immune system of the intact animal. There appears to be a specific receptor for phorbol esters on T cells, a finding that may account for the selective toxicity of phorbol esters for T cells. The mechanism of tumor promotion by phorbol esters is still largely unknown, but findings suggest an initial cell membrane interaction (often with stimulation of second messenger synthesis and increased protein kinase activity), with subsequent effects on nuclear regulatory events.