8-methoxypsoralen and UVA promote sister-chromatid exchanges

8-Methoxypsoralen (8-MOP) and long-wave UV light (UVA 365 nm) are now being use to treat vitiligo and psoriasis. Cultured human lymphocytes exposed to these agents in vitro show an increased infrequency of sister chromatid exchanges, related to 8-MOP-DNA photoadducts. Although these data raise questions regarding the biologic consequences of this therapeutic regimen, it is unknown whether 8-MOP and UVA cause mutations or extracutaneous somatic cell recombinants in vivo. Exchanges may represent cellular repair of DNA damage.