D1Z2 is a highly polymorphic DNA locus composed of a tandem of repetitive units. Its molecular constitution has been examined in 61 clonal cell lines selected at random from two lymphoblastoid cell lines (LCLs), each of which had been proliferating in vitro for several hundred days. Thirty-three of the cells were selected from an LCL derived from the blood of a person with Bloom's syndrome (BS), and the others from a normal person. A total of 20 distinctive band alterations in D1Z2 were observed, all in BS cells: appearance of a novel band(s); disappearance of a band(s), or alterations in the intensity of a band(s). Unequal sister-chromatid exchange giving rise to intra-locus mutation is considered the most plausible explanation for the accumulation of the changes detected.