Abstract
The segregation of chromosomes 13 distinguishable by Q band fluorescent polymorphisms has been studied in three families with retinoblastoma. The recombination fraction for two of these families and four families previously reported did not differ significantly from 50%. Since a high recombination fraction has been predicted from chiasma frequency between the centromere of chromosome 13 and 13q14 these results neither confirm nor refute the location of the autosomal dominant gene predisposing to retinoblastoma in 13q14. The use of fluorescent markers is not suitable for early recognition of gene carriers in families with retinoblastoma.