Elevated CD3+TCRαβ+CD4−CD8−T cells (>2.5% of CD3+ lymphocytes) was found in five unrelated patients with ALPS; two of them carried the same missense homozygous mutation (p. Phe133Val) in FAS. Increased TCRαβ DNT were also found in nine patients with other immunodeficiency disorders such as: CVID, STAT1-GOF, LRBA, ARPC1B, RIPK1, and SASH3 deficiencies. Here, ARPC1B is linked to immunodeficiency disease.