A number of investigators have now demonstrated that murine leukemia virus (MLV) vectors have a propensity to integrate around transcription start sites (TSS) [8] with a higher than expected integration frequency within 2 introns of Mds1/Evi1 gene [5], which was previously identified as a component of MDS-EVI1 translocation 3∶21 found in human acute myelogenous leukemia (AML) [9]. The gene discussed is MECOM; the disease is acute myeloid leukemia.