The retinoblastoma family of proteins including pRB, p107 and p130 undergoes cell cycle dependent phosphorylation during the mid-G1 to S phase transition. This phosphorylation is dependent upon the activity of cyclin D/cdk4. In contrast to pRB and p107, p130 is phosphorylated during G0 and the early G1 phase of the cell cycle. We observed that p130 is speci®cally phosphorylated on serine and threonine residues in T98G cells arrested in G0 by serum deprivation or density arrest. Identi®cation of the phospho-serine and phosphothreonine residues revealed that most were clustered within a short co-linear region unique to p130, de®ned as the Loop. Deletion of the Loop region resulted in a change in the phosphorylation status of p130 under growth arrest conditions. Notably, deletion of the Loop did not aect the ability of p130 to bind to E2F-4 or SV40 Large T antigen, to induce growth arrest in Saos-2 cells, and to become hyperphosphorylated during the proliferative phase of the cell cycle. p130 undergoes speci®c G0 phosphorylation in a manner that distinguishes it from pRB and p107. Oncogene (2000) 19, 5116 ± 5122.