Retinoblastoma (RB) is a rare childhood tumor which develops when both copies of the RB1 gene become mutated in developing retina, so that the amount of pRB falls below a level which may be necessary to induce terminal differentiation. RB1 mutations are necessary, but not sufficient for tumorigenesis in human developing retina. The carboxy-terminal domain is additionally required for binding to cell cycle regulatory proteins such as E2F. Regulatory proteins bind the carboxy-terminal domain of pRB. Retinoblastoma is a rare tumor that has yielded great insight into cancer, despite the phenotypic complexity of mutation of the RB1 gene in various tissues in mice and humans. The capability to precisely identify the causative RB1 mutation in each family is currently being applied with enormous implications for traditional health care, although educational and political effort is still required to get molecular diagnosis fully implemented on a global scale.