STAT5B and Splenomegaly: Due to stochastic activity of the Foxp3 promoter (43), constitutive overexpression of STAT5B-CA led to progressive splenomegaly and hematopoietic neoplasia in Foxp3YFP-Cre/YRosa26Stat5b-CA/wt mice (data not shown), similar to that after STAT5 activation in other hematopoietic compartments (44–46).