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. Here, MAP2K7 is linked to myeloproliferative disorder.