Two isoforms of the regulatory subunits of cyclic AMP (cAMP)-dependent protein kinase that bind cAMP are inversely expressed during ontogeny and cell differentiation. These cAMP-binding receptor proteins in harmony may regulate the growth of normal cells and their differentiation into nondividing states. Cancer cells can also be made to differentiate and stop growing when the functional balance of these cAMP receptor proteins is restored by treatment with site-selective cAMP analogues or by the use of an antisense oligodeoxynucleotide, suggesting new approaches to cancer treatment.