On one hand, there are phenotypic differences; Stat5a-deficient mice exhibit poor mammary function (Liu et al., 1997), reduced hematopoietic stem cell proliferation (Zhang et al., 2000) and diminished antibody class switching (Kagami et al., 2000), while Stat5b-deficient mice exhibit dwarfism (Udy et al., 1997), more pronounced lymphopenia, and greater defects in cytokine-driven lymphocyte proliferation (Moriggl et al., 1999b; Imada et al., 1998). Here, STAT5A is linked to lymphopenia.