The present invention relates to regulation of cellular proliferation. More particularly, the present invention is directed to nucleic acids encoding SAK, which is a protein kinase involved in modulation of cellular proliferation and cell cycle regulation. The invention further relates to methods for identifying and using agents, including small molecule chemical compositions, antibodies, peptides, cyclic peptides, nucleic acids, RNAi, antisense nucleic acids, and ribozymes, that modulate cell cycle regulation and cellular proliferation via modulation of SAK; as well as to the use of expression profiles and compositions in diagnosis and therapy related to cell cycle regulation and modulation of cellular proliferation, e.g., for treatment of cancer and other diseases of cellular proliferation.