Using NK cells from two patients, one with a primary immunodeficiency characterized by a homozygous mutation in CD25 (born in year 2007 and studied since she was 3 years old) and one with a homozygous mutation in STAT5b (born in year 1992 and studied since she was 10 years old), we observed that the absence of IL-2 signaling through CD25 promotes the accumulation of CD56brightCD16high NK cells, and that CD56brightCD16lo, CD56brightCD16high, and CD56dimCD16high NK cells of this patient exhibited higher content of perforin and granzyme B, and proliferation capacity, compared to healthy donors. The gene discussed is PRF1; the disease is inborn error of immunity.