The rate of chromosome breakage is age dependent in lymphocytes of adult controls.

Chromosome breaks and chromatid-type lesions from a prospective study of more than 1000 lymphocyte karyotypes from each of six controls were analysed. These lesions were more frequent in older (75 years old on average) than in younger (29 years old on average) controls, especially after 72 h cultures. All controls were found to be carriers of fragile sites. The most frequent were 3p14.3 and 16q23, especially in older controls. At least one fra (X) (q27) mitosis was found in each control. Most deletions occurred after breakage in heterochromatin or in late-replicating euchromatin. As almost all radials were either "mitotic chiasmata" or triradials (branched chromosomes), it is concluded that chromatid exchanges between non-homologous segments are very rare, and indicate chromosomal instability syndrome or recent exposure to a mutagen.