a, In an in vitro kinase assay, recombinant human Rb protein (2 μg) was incubated alone or in the presence of p38γ or CDK2–cyclin A kinases (1 μg) and 0.2 mM of cold ATP for 60 min. Interpreted MS/MS spectra demonstrating the phosphorylation of the indicated sites in Rb (in lower-case letters). The table shows the total spectral counts of the peptides where each phosphosite was identified. The data are representative of at least three independent experiments. No phosphopeptides were identified in the negative control without kinase. b, Quantitative PCR performed on liver extracts from AlbCre and AlbCre-p38γ mice. Expression was normalized to Gapdh. Data are shown as mean ± s.e.m. n = 15 mice for Cdk1, Cdk2, Cdk4 and Cdk6, and n = 6 mice for cyclins A1 (Ccna1), D1 (Ccnd1) and E1 (Ccne1). *P < 0.05; **P < 0.01; ***P < 0.001. Comparisons were made by one-way ANOVA coupled to Bonferroni’s post-tests.