(a) Northern blot analysis with a random-primed, ³²P-labeled human cyclin D1 cDNA probe in three different founder lines with low, moderate, and high cyclin D1 expression in tongue (T) and esophageal (E) tissues. (b) Tongue and esophageal epithelial protein extracts from mice expressing moderate levels of cyclin D1 were immunoprecipitated with Cdk4 or Cdk6 antibody. After a kinase reaction with [γ-³²P]ATP and GST-retinoblastoma carboxyl-terminus substrate (GST-Rb c-term) or histone H1 substrate, products were electrophoresed on a 12.5% SDS-polyacrylamide gel.