MLLT3 and acute myeloid leukemia: To assess whether CXCL12 expression by endothelial cells (ECs) or mesenchymal progenitor cells (MPCs) regulates AML cells, we transplanted c-Kit+MLL-AF9 cells into Cxcl12f/f-Tek-Cre+ (Cxcl12−/− ECs) and Cxcl12f/f-Prx1-Cre+ (Cxcl12−/− MPCs) recipient mice with Cxcl12 knocked out in ECs and MPCs, respectively (Figure 6A).