Xeroderma pigmentosum (XP) is an autosomal recessive disorder characterized by photo-induced deterioration of the skin, which often leads to the early development of skin cancers. To diagnose patients with XP and the related disorder Cockayne syndrome (CS), our laboratory has established a simple autoradiographic method that examines three cellular markers of DNA repair: unscheduled DNA synthesis (UDS), recovery of RNA synthesis (RRS) and recovery of replicative DNA synthesis (RDS). However, it is very laborious to measure the three markers using tritiated thymidine or uridine; therefore, we developed a non-isotope method for diagnosing XP and CS. Fibroblasts from the patient were labeled with bromodeoxyuridine (BrdU) instead of tritiated thymidine to measure UDS and RDS, or were labeled with bromouridine (BrU) instead of tritiated uridine to measure RRS. Incorporated BrdU or BrU could be detected using the immunofluorescence method. Moreover, we discovered a new useful marker for XP variant based on checkpoint activity. The non-radioisotope method and the new marker described here comprise an easy way to diagnose XP and CS.