The phosphorylation defective RARα mutant inhibits G1 to S phase progression in human cancer cells. Decreased BrdU incorporation by carcinoma cells expressing the phosphorylation defective RARα mutant. Cultures of G418 resistant (neo), HA tagged RARα (HA1,2), and S77A RARα mutant (RAR1-4) SCC25 clones were subjected to BrdU incorporation analysis as described in Methods. The number of BrdU positive cells was expressed as a percentage of total cells counted in ten high power fields. These experiments were performed three times with similar results. Error bars indicate SEM.