MX1 and juvenile myelomonocytic leukemia: Mx1-Cre, KrasG12D and Mx1-Cre, Nf1flox/flox mice develop a JMML-like MPN that is remarkably responsive to MEK inhibition in vitro and in vivo19, 20, 25, 26 and genetically inactivating ERK or p110α profoundly attenuates MPN in this strain27, 28.