The primary objective of this study was to understand the structural, genomic and pathophysiological impacts of two human leukemic STAT5B mutations (Kiel et al., 2014; Rajala et al., 2013), STAT5BY665F and STAT5BY665H, identified in patients with T cell leukemias (Kiel et al., 2014; Rajala et al., 2013). This evidence concerns the gene STAT5B and T-cell leukemia.