In addition, AID introduced DNA single-strand breaks within the tumor suppressor gene CDKN2B in Ph(+) ALL cells, which was sensitive to BCR-ABL1 kinase inhibition and silencing of AID expression by RNA interference.     ///    Forced expression of BCR-ABL1 in Ph(-) ALL cells and inhibition of the BCR-ABL1 kinase showed that aberrant expression of AID depends on BCR-ABL1 kinase activity.     ///     The Philadelphia chromosome (Ph) encoding the oncogenic BCR-ABL1 kinase defines a subset of acute lymphoblastic leukemia (ALL) with a particularly unfavorable prognosis. 