On the other hand, deletion of Stat5a and Stat5b has comparable effects on some physiological processes, such as eosinophil recruitment (Kagami et al., 2000), and the most dramatic phenotypes, such as infertility, anemia and perinatal lethality, are evident only in mice lacking both paralogs, which implies redundancy and/or cooperativity (Teglund et al., 1998; Socolovsky et al., 1999; Cui et al., 2004). This evidence concerns the gene STAT5B and anemia (phenotype).