Pre-B cell receptor–mediated up-regulation of BCL6 inhibits proliferation by transcriptional repression of Myc. Pre-B ALL cells from Bcl6^+/+ and Bcl6^−/− bone marrow were transduced with retroviral expression vectors for CD8/μ-chain or CD8 alone. The relative change of the percentage of CD8⁺ cells over time was measured by flow cytometry (A; n = 3). To identify BCL6-target genes, ChIP-on-chip analysis was performed for 3 human BCR-ABL1 pre-B ALL cell lines (BV173, Tom1, Nalm1) and 2 human diffuse large B-cell lymphoma cell lines (OCI-Ly1 and OCI-Ly7) as positive controls. High expression levels of BCL6 were induced by treatment of BCR-ABL1 pre-B ALL cell lines with 10μM STI571 for 24 hours (STI571). Recruitment to CCND2 and MYC (see supplemental Figure 3) promoters are shown and to the HPRT promoter as a negative control. The ChIP-on-chip data for Myc in Nalm1 cells is also published in Duy et al.⁹  (B) BCR-ABL1 pre-B ALL cells were transduced with 4-hydroxy-tamoxifen (4-OHT)–inducible vectors for BCL6-ER^T2 and ER^T2 empty vector control. Myc mRNA (C; n = 2) and protein (D; n = 2) levels on 4-OHT induction were measured by quantitative RT-PCR and Western blot, respectively. Induction of BCL6 also resulted in up-regulation of Cdkn1b (p27) protein levels (D). Overexpression of Myc was sufficient to partially rescue BLNK/BCL6-mediated inhibition of proliferation (E): Blnk^−/−BCR-ABL1 pre-B ALL cells were transduced with Myc-Puro or a Puro-empty vector control and subjected to antibiotic selection. Subsequently, pre-BCR signaling was reconstituted by retroviral expression of BLNK-GFP or a GFP empty vector control and proliferation of the Myc-Puro versus Puro-transduced cells was measured as relative change of the percentage of BLNK-GFP⁺ or GFP⁺ cells (E; n = 2).