Chimeric myeloid cell hyperactivation syndrome has been described in humanized NSG-SGM3 (NOD.Cg-PrkdcscidIl2rgtm1Wjl Tg(CMV-IL3,CSF2,KITLG)1Eav/MloySzJ)6,10 and more recently NOG-EXL mice11 and is characterized by multiorgan infiltrates of hemophagocytic histiocytes, anemia, increased eosinophilopoiesis, and, in NSG-SGM3 mice, mastocytic infiltration of the pancreas. The gene discussed is IL3; the disease is anemia.