Thiopurine prodrugs azathioprine (Aza) and 6-mercaptopurine (6MP) figure in the World Health Organization top one hundred list of essential medicines and are prescribed for the treatment of leukemia, autoimmune conditions, and as immunosuppressants. Although they are highly effective medications, long-term treatment of patients with thiopurine prodrugs is associated with a 10-fold increase in basal cell carcinomas and a 65-to 250-fold increase in squamous cell carcinomas. These side effects are induced by exposure to sunlight, which is thought to originate from the ability of thiopurine derivatives to absorb UVA radiation (320-400 nm). Indeed, UVA irradiation of cells treated with Aza and 6MP causes photochemical damage, increasing oxidatively generated damage of both the genome and the proteome. The photochemical properties of 6-thioguanosine (6tGuo), a clinically-active metabolite of these prodrugs, have received widespread attention because is thought to be the root cause of the mutagenic damage.