Structural or functional inactivation of the retinoblastoma gene product (pRB) is inevitably associated with human malignancy.Mechanistically, the tumor suppressor function ofpRb is linked to its ability to regulate cell cycle and differentiation.In cells committed tomuscle fate pRb binds proliferation and differentiation through interaction with transcription factors of E2F and MyoD families.pRb shows different affinity to distinct E2Fs, however, mechanisms of the pRb-E2Fs interaction are not fully investigated.We have found here that pRb carrying a small deletion at the end of the T antigen binding region (△S/N) and unable to interact with large T antigen still keeps the ability to induce acute cell cycle block, stable prolongation of cell cycle at G0/G1 and G2/M phases and suppress growth of tumor cells.△S/N shows increased affinity to E2F4, binds hyperphosphorylated forms of E2F4 and produces stable increase in the level of E2F4.