Positive selection of CD4⁺CD8⁺ thymocytes from H-2^k 2C mice is associated with upregulation of CD2 expression. Thymocytes from the indicated mice were stained and analyzed as described in Fig 1. FL1 = CD8 log fluorescence; FL2 = CD4 log fluorescence; X-axis = log fluorescence; Y-axis = relative cell number. The fluorescence level of the indicated molecule on gated CD4⁺CD8⁺ thymocytes with (dotted line) and without (solid line) the β₂-microglobulin null mutation (β₂m^o) is shown.