This first thematic issue, of the Advances in Immunology series, highlights the remarkable new insights into the mechanisms that govern development and function of T cell lineages. Recent developments in the understanding of the genetic and epigenetic mechanisms that regulate development of the two major T cell lineages will have a fundamental impact on a number of research fields -immunology, cell biology, hematology and stem cell research. All of these groups have a vested interest in comprehending issues such as stem cell self renewal, progenitor plasticity, lineage commitment and...