Corrected mononuclear cells activate and proliferate in response to mitogen. PBMCs from (A) an aged-matched normal and (B) SCID-X1 dog R2202 at 46 weeks after injection were treated with recombinant human IL-2, and STAT5 phosphorylation (pSTAT5) was assessed by flow cytometry. PBMCs from the SCID-X1 dog display STAT5 phosphorylation approximately 20% of that of normal. (C-E) Treatment with PHA results in cell-cycle entry of PBMCs derived from a (C) normal unaffected dog or (D-E) foamy virus–treated SCID-X1 dogs X349 at 22 weeks and R2202 at 24 weeks after treatment, respectively. PBMCs proliferate in response to mitogen at ∼25% to 32% of that of the control.