Purpose: In Rd1 mice, a PDE6ß mutation is responsible for the rapid loss of photoreceptors. We observed re-expression of cell cycle proteins during early stages of retinal degeneration and the deletion of Bmi1 markedly delayed photoreceptor death in Rd1;Bmi1-/- mice. The present study characterizes the link between the expression of CDKs and the apoptotic process in Rd1 photoreceptors.Methods: CDK expression levels were evaluated by immunostaining of wild-type, Rd1 and Rd1;Bmi1-/- eye sections. The role of CDKs in retinal degeneration is currently being investigated by treating Rd1 retinal explants with CDK inhibitors, and by injecting roscovitine-containing micelles into the vitreous of P10 Rd1 mice.Results: We show that some Rd1 photoreceptors express CDK4 already at P9, and that the number of CDK4-positive cells increases more than 6-fold by P11. CDK2 and CDK6 are also expressed in the mutant outer nuclear layer (ONL), however to a lesser extent than CDK4. Concomitant with the expression of CDKs, the apoptotic process in Rd1 photoreceptors is detected by TUNEL staining. Co-localization analyses suggest that CDK expression precedes photoreceptor cell death since TUNEL-single-positive cells are rarely detected at P9, and double-positive as well as TUNEL- or CDK4-single-positive cells are all present in P11 Rd1 retinas. The wild-type ONL does not contain any TUNEL- or CDK4-positive cells. Interestingly, Bmi1 deletion downregulates CDK4 expression in P12 Rd1;Bmi1-/- retinas, and influences the accumulation of cGMP in Rd1 retinas. More cGMP is detected in the P11 Rd1;Bmi1-/- ONL than in the Rd1 ONL, while it is strongly reduced at P15. To better characterize the link between CDK expression and retinal degeneration, current experiments include the analysis of CDK inhibition in Rd1 retinal explants and in mouse eyes injected with roscovitine-containing micelles.Conclusions: The time-course of cell cycle protein expression may be related to early events of the apoptotic process in Rd1 photoreceptors. Moreover, the loss of Bmi1 seems to interfere with the first stages of retinal degeneration and to influence the expression of CDK4. Further experiments will determine whether the deletion of Bmi1 prevents cell death through a direct CDK inhibition.