The understanding of the molecular biology of human cancer has advanced rapidly in the last decade, in part due to discoveries in the rare, pediatric ocular tumor, retinoblastoma. RB studies have led to recognition of a class of human genes, the tumor suppressor genes, that are critical in the initiation and progression of the malignant process. Mutations in the RB1 gene initiate RB and other specific tumors. They may also contribute to progressive stages of many other malignancies. The protein product of RB1 (p110RB1) is a basic regulator of the cell cycle. In the absence of normal protein, the cell proceeds to the next cell division without the potential to become quiescent. Understanding the genetics of RB has benefited the patients, as the precise identification of the RB1 mutations in families has led to accurate prediction of individuals at risk for RB tumors. It seems unlikely, in the foreseeable future, that direct genetic manipulation of mutant RB1 genes will play a role in therapy, but complete understanding of the function of p110RB1 may eventually allow exploitation of its powerful antiproliferative effect. Other molecular genetic events in addition to RB1 mutations are documented in RB tumors, and may play a critical role in the full malignant phenotype. The oncogene, N-myc, is amplified in some RB tumors and is expressed in normal fetal retina. The cytogenetic abnormality, i(6p), is almost unique to RB tumors. The molecular and tissue-specific roles of these abnormalities are not yet known. Many RB tumors also acquire excessive expression of the cell surface membrane glycoprotein, p170, linked to multidrug resistance, whether or not the RB tumor has been exposed to chemotherapy. We anticipate that ways to avoid or counteract the drug resistance of excessive p170 expression will be developed for other pediatric tumors and eventually will be applied to chemotherapy for RB patients.