Armed with the tools of the model isolated cell lines of the fasciculate and the adrenocortical carcinoma 494 cells, with the techniques of double-labeled 3H/C14 radioisotopes, and with the information that ACTH control of steroidogenesis is indirect, occurring through the mediation of its second messengers, cyclic GMP, and cyclic AMP, we were in a good position to define the ACTH-dependent rate-limiting step of steroidogenesis (Wickramsinghe et al., 1969; Sharma, 1970a,b,c). This evidence concerns the gene POMC and adrenal cortex carcinoma.