A candidate for the retinoblastoma (Rb) predisposition gene, known as 4.7R, has recently been cloned. Authentication of this gene depends on abnormalities detected in either the genomic sequence or transcription products (or both) in Rb tumours and cell lines. We report a constitutional chromosome translocation, with a breakpoint lying within the putative retinoblastoma gene, from a patient with retinoblastoma; there is an associated small intragenic deletion. This structural rearrangement, which confers susceptibility to tumour formation, provides strong evidence that the 4.7R cDNA represents the Rb gene.