CD34 and macrophage activation syndrome: Since symptoms of secondary hemophagocytic lymphohistiocytosis (HLH) and/or macrophage activation syndrome (MAS) were described for NSG-SGM3 mice after engraftment with human CD34+ stem cells (30–32), we measured the numbers of erythrocytes, reticulocytes and the level of hemoglobin in the blood of the humanized mice every two weeks post engraftment (Figures 1H–J).