Chromosomal translocations that lead to the formation of hyper-active JAK2 fusion proteins such as TEL-JAK2, BCR-JAK2, and PCM1-JAK2 signal through STAT5 and are frequently detected in various leukemia subtypes [for references see reviews by Valentino and Pierre (2006) and Ghoreschi et al. (2009) [19], [20]. This evidence concerns the gene STAT5B and leukemia.