CXCR4 and leukemia: Consistent with this, when GFP+ (sgRNA-expressing) leukemia cells were sorted prior to the homing assay, there was no significant difference in the percentage of GFP+ cells in the bone marrow 24 h after transplantation (Figure S4B), suggesting that CXCR4 is not critical for MLL-AF9 leukemia cell homing to the bone marrow.