CXCR4 and leukemia: We sequentially transduced MLL-AF9 leukemia cells with Cxcr4 sgRNA_b coexpressing tRFP657 and Cxcr4 cDNAs coexpressing GFP, which resulted in restored CXCR4 expression on the cell surface (Figures 7A and 7B; Figure S7B).