The CD3ε BRS is not required to conceal the PRS in absence of TCR–CD3 engagement. A, Schematic representation of the amino acid sequence of murine CD3ε and its domains with either the WT (top) or the substitute (bottom) BRS sequence (EXTRA, extracellular; TM, transmembrane; BRS, basic amino acid-rich stretch; PRS, proline-rich sequence; ITAM, immunoreceptor tyrosine-based activation motif). B, Thymocytes from mice transgenic for either CD3ε BRS WT or BRS-Substitute were lysed and subjected to the CD3-PD assay to assess background, basal, and 2C11-stimulated open-CD3 levels as previously described. C and D, Values of basal open-CD3 as a percent maximum of 2C11 stimulation (C) and of open-CD3 induction achieved by 2C11 stimulation over basal (D) are plotted for six independent experiments. The mean ± SE is displayed representing the amount of open-CD3 found in thymocytes from each mouse type. Paired t tests were run to calculate p values, which indicate that basal and open-CD3 induction in CD3ε BRS WT versus BRS-Substitute thymocytes are not statistically different.