CSF3 and neutropenia: Standard clinical criteria for RD include 1) absence, or very low number of T cells (CD3 T cells <300/microliter); 2) no, or very low (<10% of lower limit of normal) T cell function (as measured by response to PHA; 3) severe neutropenia (absolute neutrophil count <200/microliter) that is typically unresponsive to G-CSF; and 4) sensorineural deafness and/or absence of granulopoiesis at bone marrow examination and/or a deleterious AK2 mutation14.