In an analysis of mutations in the RB1 gene in three patients, selected at random, who had a positive family history of tumours, we identified mutations, in constitutional cells, involving exons 3, 13 and 17 of the RB1 gene. We used SSCP and PCR sequencing to screen affected individuals and other members of their families. In two cases the mutations were 2 bp and 1 bp deletions identified in exons 3 and 17 respectively. The third mutation was a 1 bp insertion in exon 13. All three mutations lead to the generation of downstream premature stop codons as a result of frameshift changes, although the mutation in exon 3 possibly affects the splicing mechanism. The sites within the RB1 gene where these mutations occur contain interspersed repetitive DNA sequences, direct and inverted repeat sequences and/or dyad symmetrical elements suggesting that these areas promote the appropriate local sequence environment for the generation of deletions and insertions in the RB1 gene.