Hum. Genet. 51, 73--78 (1979) © by Springer-Vedag 1979 Absence of Chromosome Breakage in Patients with Retinoblastoma Louise A. Knight 1, H. Allen Gardner l* , and Brenda L. Gallie 2 1 Division of Cytogenetics, Department of Pathology, Toronto General Hospital and The University of Toronto, Toronto, Canada 2Department of Ophthalmology, Wellesley Hospital, and the University of Toronto, Toronto, Canada Summary. Mixed lymphocyte cultures were employed to assess the degree of spontaneous chromosome fragility in patients with retinoblastoma. There was no difference between the patients and their controls. If chromosome in- stability plays a role in the inherited tumour, more sensitive methods need be employed to elucidate it. Introduction A significantly elevated frequency of aneuploidy, chromatid-type aberrations, and stable abnormalities of the chromosomes has been reported by Czeizel et al. (1974) in 12 children suffering from retinoblastoma. The authors interpreted these results as reflecting increased fragility of the patient's somatic chromosomes and pointed out that chromosome instability is also present in patients with Bloom's Syndrome, Fanconi's anaemia, Louis-Bar Syndrome (ataxia telangiec- tasia) and xeroderma pigmentosum (German, 1972), all of which cary an increased risk of developing malignant disease. Although the cytogenetic instabil- ity of these four syndromes has been confirmed in many laboratories, that associated with retinoblastoma has not. More recently, Wiechselbaum et al. (1978) showed that skin fibroblasts from 6 patients with hereditary retinoblastoma appear to be more sensitive to the effects of X-rays than do fibroblasts from patients with sporadic retinoblastoma or normal controls. The sensitivity was intermediate between that observed in ataxia telangiectasia and the normal control. In order to elucidate more fully the role of chromosome fragility, 12 patients with retinoblastoma were studied. * To whom offprint requests should be sent 0340-6717/79/0051/0073/$ 01.20