The BCR-ABL1 kinase expressed in acute lymphoblastic leukemia (ALL) drives malignant transformation of pre-B cells and prevents further development. We studied whether inhibition of BCR-ABL1 kinase activity using STI571 can relieve this differentiation block. STI571 treatment of leukemia patients induced expression of the Ig L chain-associated transcription factors IRF4 and SPIB, up-regulation of RAG1 and RAG2, Cκ and Cλ germline transcription, and rearrangement of Ig κ L chain (IGK) and Ig λ L chain (IGL) genes. However, STI571-treated pre-B ALL cells expressed λ L, but almost no κ L chains. This could be explained by STI571-induced rearrangement of the κ-deleting element (KDE), which can delete productively rearranged Vκ-Jκ joints. Amplifying double-strand breaks at recombination signal sequences within the IGK, KDE, and IGL loci revealed a coordinated sequence of rearrangement events induced by STI571: recombination of IGK gene segments was already initiated within 1 h after STI571 treatment, followed by KDE-mediated deletion of Vκ-Jκ joints 6 h later and, ultimately, IGL gene rearrangement after 12 h. Consistently, up-regulation of Cκ and Cλ germline transcripts, indicating opening of IGK and IGL loci, was detected after 1 and 6 h for IGK and IGL, respectively. Continued activity of the recombination machinery induced secondary IGK gene rearrangements, which shifted preferential usage of upstream located Jκ- to downstream Jκ-gene segments. Thus, inhibition of BCR-ABL1 in pre-B ALL cells 1) recapitulates early B cell development, 2) directly shows that IGK, KDE, and IGL genes are rearranged in sequential order, and 3) provides a model for Ig L chain gene regulation in the human.