It has been proposed that differential kinetics of CD4/CD8 coreceptors regulate fate choice of selected thymocytes. Sustained signals by interaction between MHC class II and TCR/CD4 is required for commitment to the CD4 helper lineage. Although prematurely terminated MHC-TCR/CD4 interaction in transgenic mouse models results in lineage redirection, it is unclear whether CD4 expression is actively maintained by endogenous cis-elements to facilitate prolonged signaling under physiological conditions. In this article, we show that sustained CD4 expression in postselection thymocytes requires an intronic sequence containing an uncharacterized DNase I hypersensitivity (DHS) site located 39 to the silencer. Despite normal CD4 expression before selection, thymocytes lacking a 1.5-kb sequence in intron 1 including the 0.4-kb silencer and the DHS, but not the 0.4-kb silencer alone, failed to maintain CD4 expression upon positive selection and are redirected to the CD8 lineage after MHC class II-restricted selection. Furthermore, CpG dinucleotides adjacent to the DHS are hypermethylated in CD8+ T cells. These results indicate that the 1.5-kb cis-element is required in postselection thymocytes for helper lineage commitment, presumably mediating the maintenance of CD4 expression, and suggest that inactivation of the cis-element by DNA methylation may contribute to epigenetic Cd4 silencing.