pRB Transcription Factor

When pRB is active in the nucleus, it binds E2F, a protein transcription factor that controls expression of a large gene set encoding genes that move the cell through the cycle. When bound to pRB, E2F is inactive, and the cell does not move from G1 into S. If pRB is inactivated by the addition of phosphate groups (P), E2F is released, binds to promoter regions, gene expression begins, and the cell moves into S and completes the cycle. If pRB is missing or mutated, E2F is always active, and the cell grows and divides continuously, forming a cancerous tumor.