Detection of p16/INK4a, p16β/INK4a, and p15/INK4b mRNA by RT-PCR and hybridization with an internal probe. After electrophoresis, PCR products were transferred to nylon filters and hybridized with probes corresponding to exon 1 of p16/INK4a, p16β/INK4a, p15/INK4b, and rat GAPDH cDNA. Hybridizing bands were visualized by Phospholmager.