STAT5B and acute lymphoblastic leukemia: In pediatric T-ALL, activating mutations of JAK1 and JAK3 and other genes involved in JAK/STAT signaling, such as EP300, STAT5B, IL7R and PTPN11, have been associated with an increased JAK/STAT signaling (31, 32) and, in the presence of IL7R mutations, with an increased resistance to steroid therapy (33).