There are increasing reports of multiple different types of somatic mosaicism detected in patients with inherited and non-inherited disorders. The characteristics of several of the major types of mosaicism will be outlined, and contrasted with somatic mosaicism, which is the focus of this article. This review examines examples of somatic mosaicism due to differences in DNA sequence arising from in vivo site specific reversion to normal of inherited mutations in humans. While several known mechanisms of reversion are evident in a number of these examples, they are not in some others. The possible significance of the role of selection, particularly in view of recent results of gene therapy, is discussed.