The critical moment of regulation when transition of the cell from the G1 to the S phase takes which is decisive for its division is the so-called R-point (restriction point). The disorder of molecular mechanisms controlling the passage of the cell through this site is an associated manifestation of most tumourous processes. A considerable part of this mechanism is the RB protein, binding transcription factors needed for the activation of genes effective in the S phase. Its binding capacity depends on the extent of phosphorylation and on the activity of the appropriate kinases and their inhibitors.