In mammals, STAT5 is encoded by two adjacent genes, Stat5a and Stat5b. To avoid the confounding lymphopenia, anemia, and inflammatory disease associated with ablation of both paralogs, we generated a series of mice with decreasing numbers of STAT5 alleles, ranging from four (i.e., two copies each of Stat5a and Stat5b) to one (i.e., one copy of either Stat5a or Stat5b; Villarino et al., 2016; Fig. S1 A). Here, STAT5B is linked to lymphopenia.