[en] With low dose levels of ionizing or ultraviolet radiation, the number of initiation events exceeds the number of tumors that grow to a detectable size. Ionizing radiation, which is a complete carcinogen, appears to be a more effective initiator than an enhancer or promoter. However, the initiation and promotion aspects of ionizing radiation have been studied in very few organ systems. In the case of UVR, with or without photosensitizers such as psoralens, the requirement of a relatively large number of exposures for carcinogenesis suggests that the expression of the initiated cells as frank tumors requires a number of events spread out over the time of the development of the tumor. Both ionizing and ultraviolet radiation are, perhaps, underutilized as tools for probing the mechanism of both initiation and promotion