Whi5 is a transcriptional regulator in the budding yeast, notably in the G1 phase. It plays an important role in cell size control in G1 phase, similarly with Retinoblastoma (Rb) protein in human, although the two have no similarity in sequence Whi5 is an inhibitor of SBF (SCB binding factor), which is involved in the transcription of G1-specific genes. Cln3 promotes the disassociation of Whi5 from SBF, and its disassociation results in the transcription of genes needed to enter S phase.