Stat5a and Stat5b proteins have essential and nonessential, or redundant, roles in cytokine responses. - Teglund,S.; McKay,C.; Schuetz,E.; van Deursen,JM.; Stravopodis,D.; Wang,D.; Brown,M.; Bodner,S.; Grosveld,G.; Ihle,JN. (Cell, 1998)