Proliferative responses of thymocytes from Jak3^+/+ mice (+/+), control-treated Jak3^−/− mice (−/−), and IL-3–treated Jak3^−/− littermates (−/− IL-3). Jak3^−/− thymocytes do not respond to T-cell mitogenic and Jak3-dependent cytokine stimuli. Thymocytes were stimulated with mitogen/cytokine combinations as described in Materials and Methods, and their proliferation was measured by incorporation of [³H]-thymidine. Values are average stimulation indices ± SD (n = 2 in each group).