Repressor of G1 transcription; binds to SCB binding factor (SBF) at SCB target promoters in early G1; phosphorylation of Whi5p by G1 cyclin (Cln)/Cdc28p relieves repression and promoter binding by Whi5p, drives nuclear export, and contributes to both 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; expression is repressed in M/early G phase; analogous to metazoan Rb