LEF1 and acute lymphoblastic leukemia: Relevant to the pathogenesis of T‐ALL, this analysis identified recurrent deletion of CDKN2A, CDKN2B, STIL, LEF1, PTEN, 9q34.11–13, FBXW7, 1p33/STIL and gain of MYB.6, 7, 8, 9, 10, 11, 12, 13