Although the mechanisms responsible for such decreased leukocyte-subset counts in blood of T/NK-LGLL patients remain unknown, the presence of cytopenias (i.e., neutropenia, anemia, and thrombocytopenia) in LGLL, has been associated with an altered production of hematopoietic cells in the BM—similarly to what occurs in children with primary immunodeficiencies (PID) carrying STAT3 germinal mutations [9,20,35]—and/or an accelerated cell turn-over in the periphery [36,37]. Here, STAT3 is linked to inborn error of immunity.