[en] The study of three diseases, ataxia telangiectasia, basal cell naevus syndrome and retinoblastoma, has yielded very interesting information at the clinical level. In one, AT, there is some understanding of the possible mechanisms of susceptibility to malignant disease at the cellular, chromosomal and molecular level, but so far this is very incomplete. There is some suggestion that in retinoblastoma similar information may be beginning to emerge, but in the case of basal cell naevus syndrome there is not even a beginning of an understanding of the mechanism. The subject is reviewed under the following headings: radiosensitivity at the clinical level, the cellular level and the chromosome level; relationship between induced and spontaneous chromosome damage. (U.K.)