γ_c-transduced mice exhibited IL-2–induced proliferation of fresh splenocytes, whereas mock-transduced mice did not. ³H-thymidine incorporation assays were performed as described in Materials and Methods for wild-type mice (A), γ_c-deficient mice (B), or in γ_c-deficient mice that received a bone marrow transplant from wild-type mice (C) or γ_c-deficient bone marrow that was transduced with human γ_c (D) or mock-transduced (E). Shown are pooled data from 3 experiments; each pair of connected points represents an individual mouse.