Full context:   Entry into S-phase in both yeast and animals is controlled by the levels of two opposing regulators. The networks regulating these transcription factors are double-negative feedback loops and positive feedback loops in both yeast and animals. Additional regulation of the regulatory network for the G1/S checkpoint in yeast and animals includes the phosphorylation/de-phosphorylation of CDK-cyclin complexes. The sum of these regulatory networks creates a hysteretic and bistable scheme, despite the specific proteins being highly diverged. For yeast, Whi5 must be suppressed by Cln3 phosphorylation for SBF to be expressed, while in animals Rb must be suppressed by the Cdk4/6-cyclin D complex for E2F to be expressed. Both Rb and Whi5 inhibit transcript through the recruitment of histone deacetylase proteins to promoters. Both proteins additionally have multiple CDK phosphorylation sites through which they are inhibited. However, these proteins share no sequence similarity.