Deletions and other genome rearrangements can be caused by radiation and are associated with carcinogenesis and inheritable diseases. The pink-eyed unstable (p(un)) mutation in the mouse is caused by a gene duplication and reverts to wild type by deletion of one copy. Reversion events in the mouse embryo were detected as black spots on the fur of the animals or microscopically as partially black hair in a background of colorless hair. The frequency of partially black hair was increased by x-rays at very low doses. A linear dose-response relation was found between 1 and 100 centigray.