Repressor of G1 transcription; binds SCB binding factor (SBF) at SCB target promoters in early G1; phosphorylation by G1 cyclin/Cdc28p relieves repression and promoter binding by Whi5p, drives nuclear export, and contributes to the determination of critical cell size at START and cell fate; dephosphorylated and re-imported into the nucleus post-START during the starvation response, reversing commitment; repressed in M/early G phase; autophagy regulator and Atg1p substrate; functional Rb homolog