B2M and acute lymphoblastic leukemia: Transplanting the KMT2A-MLLT3 expressing cells into different immunodeficient mouse models led to AML in NSGS mice and AML, B-ALL or mixed lineage leukemia in NSG and NSG beta 2 microglobulin (NS-B2M) mice showing that the immune microenvironment plays an important role in leukemia development.