“…Chromatid gaps were the commonest aberration, and chromatid exchange the rarest, being recorded in only about 01°,% of cells. A high percentage of chromatid aberrations was recorded in patients after irradiation (Court Brown, Buckton, and McLean, 1965), after exposure to drugs (Ryan, Boddington, and Spriggs, 1965;Cohen, Marinello, and Back, 1967), and after viral infections (Nichols, 1966). It also occurs in certain congenital syndromes such as Fanconi's anaemia (Swift and Hirschhorn, 1966) and Bloom's syndrome (Sawitsky, Bloom, and German, 1966).…”