Figure 2: Cytogenetic manifestations. Giemsa-stained metaphases from 72-h-cultures of cells from the girl with Bloom syndrome. A: Doublesymmetrical quadriradial figure (arrow) representing a rearrangement between homologous chromosomes from a PHA-stimulated whole blood culture of peripheral blood lymphocytes. B: Characteristic Harlequin chromosomes (arrows) in a metaphase from a PHA-stimulated peripheral blood lymphocyte culture exposed to BrdU, indicating a greatly increased rate of sister chromatid exchanges (SCEs). C: A metaphase from an EBV-transformed lymphoblastoid cell line showing only occasional SCEs (arrow) suggesting a normal SCE rate and somatic reversion of the line.