Although the translocation was later found in less than 1% of T-ALL, somatic activating mutations in Notch1 receptor were detected in over 50% of human T-ALL cases [2] and 74% of tumors in a mouse leukemia model [8], showing that overexpression of activated Notch1 is indeed tumorigenic [9]. This evidence concerns the gene NOTCH1 and acute lymphoblastic leukemia.