Acts as a tumor suppressor. Inhibits DNA synthesis. Its ability to inhibit oncogenic transformation is mediated through its association with RB1. Plays a role in the expression of genes required for the G1/S transition. {ECO:0000269|PubMed:15538385, ECO:0000269|PubMed:16730350}. This is the function of LIN9 (lin-9 DREAM MuvB core complex component, Ensembl gene identifier ENSG00000183814).