UV-light induced sister chromatid exchanges in xeroderma pigmentosum lymphocytes.

Cultured lymphocytes from 9 patients with clinically different types of xeroderma pigmentosum were exposed to ultraviolet light at 24 h. An increased rate of sister chromatid exchanges were observed in 6 patients (128--148% increase in three, 34--51% in three), but not in three patients with deSanctis-Cacchione syndrome (xeroderma pigmentosum with mental defect), compared to simultaneously cultured controls. A positive result could be useful as preliminary cytogenetic diagnostic test. The results are interpreted as an expression of UV-light induced chromosomal instability due to impaired DNA repair.