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In Philadelphia chromosome-positive (Ph+) leukemia BCR/ABL induces the leukemic phenotype. Targeted inhibition of BCR/ABL by kinase inhibitors leads to complete remission. However, patients with advanced Ph+ leukemia relapse and acquire resistance, mainly due to point mutations in BCR/ABL. The ‘gatekeeper mutation’ T315I is responsible for a general resistance to small molecules. It seems not only to decrease the affinity for kinase inhibitors, but to also confer additional features to the leukemogenic potential of BCR/ABL. To determine the role of T315I in resistance to the inhibition of oligomerization and in the leukemogenic potential of BCR/ABL, we investigated its influence on loss-of-function mutants with regard to the capacity to mediate factor independence. Here, we show that T315I (i) requires autophosphorylation at tyrosine 177 in the BCR-portion to mediate resistance against the inhibition of oligomerization; (ii) restores the capacity to mediate factor-independent growth of loss-of-function mutants due to an increase in or activation of ABL-kinase; (iii) leads to phosphorylation of endogenous BCR, suggesting aberrant substrate activation by BCR/ABL harboring the T315I mutation. These data show that T315I confers additional leukemogenic activity to BCR/ABL, which might explain the clinical behavior of patients with BCR/ABL–T315I-positive blasts. [ABSTRACT FROM AUTHOR]