Figure 2.

Regulation of G₁ cell cycle progression through ERK signaling. In response to growth factor stimulation, ERK is phosphorylated and activated. Activated ERK causes enhanced expression of immediate early genes, including the AP‐1 protein family. Subsequently, delayed early genes, including cyclin D, are induced. The Cyclin D–CDK4/6 complex then initiates retinoblastoma (Rb) phosphorylation, which activates the E2F family of transcription factors and induces expression of target genes, including cyclin E. The cyclin E–CDK2 complex further phosphorylates Rb and thus activates the E2F family. This positive feedback loop drives the cells to S phase entry.