The control of CD4 gene expression is essential for proper T lymphocyte development. Signals transmitted from the T-cell antigen receptor (TCR) during the selection process are believed to be linked to the regulation of CD4 gene expression during specific stages of T cell development. Thus, the control CD4 gene expression is an ideal model system for studying the molecular mechanisms that drive T cell development. Here, I discuss the characterization of transcriptional control elements in the CD4 locus and the factors that mediate their function. The study of these elements has led to significant insights into the mechanisms in which the T lymphocyte develops it mature functional characteristics.