Developing αβ T cells choose between the helper and cytotoxic lineages, depending upon the specificity of their T cell receptors for MHC molecules. The expression of the CD4 co-receptor on helper cells and the CD8 co-receptor on cytotoxic cells is intimately linked to this decision, and their regulation at the transcriptional level has been the subject of intense study to better understand lineage choice. Indeed, as the fate of developing T cells is decided, the expression status of these genes is accordingly locked. Genetic models have revealed important transcriptional elements and the ability to manipulate these elements in the framework of development has added a new perspective on the temporal nature of their function and the epigenetic maintenance of gene expression. We examine here novel insights into epigenetic mechanisms that have arisen through the study of these genes.