The thymus is a primary lymphoid organ present in all vertebrates, and is essential for the production of most T cells. Close examination of athymic mice and humans has however revealed that small numbers of T cells can be detected in individuals lacking a thymus. The T cells observed in athymic individuals exhibit unusual phenotypic properties in comparison to conventional thymus-derived T cells, and at present the developmental origin of these extrathymic T cells is unclear. We aim to determine what role somatic mutation, either at the T cell receptor locus, or elsewhere in the genome, plays in the development of these extrathymic T cells. Understanding the development of thymus-independent T cells may provide clues as to the evolutionary history of thymic T cell restriction, and help us to understand the essential signals provided by the thymus to support conventional T cell development.