MX1 and leukemia: We then hypothesized that the AMP population observed in Runx1+/R188QMx1-CreCbfb+/56M mice is functionally different from that observed in Mx1-CreCbfb+/56M mice with loss of leukemia-initiating ability, similar to that observed in Runx1fl/flMx1-CreCbfb+/56M mice (18).