Chinese Hamster Ovary fibroblasts (CHO-K1) have shown different protein contents when undergoing differentiation by 3',5'-cyclic adenosine monophosphate (cAMP), which is known to induce reverse transformation (RT) from malignancy to fibroblast-like characteristics. The mass spectrometry (MS) investigation here reported about the behavior of CHO-K1 cells before and after exposure to cAMP reveals a change in the composition of nuclear proteins associated to an inhibition of the protein expression. Possible implications of this finding on the control of cell reverse transformation are discussed.