BCL6 as biomarker to identify pre-BCR⁻ and pre-BCR⁺ cases of human ALL. A schematic summarizing normal cytokine-dependent survival signaling in pro-B cells (pre-BCR⁻) and pre–BCR-dependent survival signaling in pre-B cells (A). SRC and BTK are targets of dasatinib, BTK is a target of ibrutinib, and PI3Kδ is the target if idelalisib (A), which are being used in clinical trials for patients with mature B-cell lymphoma. Functional assays to measure tonic pre-BCR signaling in ALL patient samples may not be practical for diagnostic purposes. Given that tonic pre-BCR signaling results in strong expression of BCL6,³¹  immunostainings for BCL6 and immunoglobulin µHC expression represents a feasible alternative to rapidly identify patients that might benefit from treatment with inhibitors of pre-BCR signaling (eg, ibrutinib, idelalisib). (B) Double stainings for BCL6 (brown; D8, Santa Cruz Biotech) and µHC (red; G20-127, BD Biosciences) were performed on paraffin-embedded sections from bone marrow clots. (B) Two patient-derived ALL cases were studied and counterstained with hematoxylin and eosin (100× magnification).⁶⁶  One pre-BCR⁻ ALL case (BCR-ABL1, left) and 1 pre-BCR⁺ ALL case (TCF3-PBX1, right) is shown. Panel C shows a western blot analysis of BCL6 expression (D8; Santa Cruz Biotech) in pre-BCR⁻ (left) and pre-BCR⁺ (right) and ALL cells, with and without treatment with the dual ABL1/SRC-BTK inhibitor dasatinib (25 nmol/L for 24 hours), using β-actin (ab8227, Abcam) as loading control (C).