Abstract

The differentiation of T lymphocytes, the cells that are responsible for cell‐mediated immunity, takes place within the thymus, but details of the developmental pathway have long been obscure. Although distinct subpopulations of thymocytes had been characterized, they acted like separate developmental streams rather than sequential differentiation stages. Recently, a minor subgroup of thymocytes representing an earlier blast population has been identified, offering the hope that the key developmental events will be discerned by focus on this new population of precursor cells.