Second, it is evident that the considerably low frequency of SD cells for SNRPN characterizing the cancer cases cannot be attributed to shortening of the duration of the S-phase since at the same time these same samples revealed a significant increase in the frequency of SD cells for CEN15 and RB1. Similarly, while an increase in the S-phase duration may explain the increase in the frequency of SD cells for CEN15 and RB1, it fails to explain the dramatic decrease in the frequency of SD observed for the SNRPN locus. This evidence concerns the gene RB1 and cancer.