PIK3CD and inborn error of immunity: Noteworthy, the evidence for T cell exhaustion has also been found in other inborn errors of immunity (IEI) characterized by impaired T cell functions, such as proliferation and cytokine expression, and increased susceptibility to apoptosis, in which genetic underpinnings predispose to immune dysregulation, such as variants in LRBA (Lipopolysaccharide (LPS) Responsive Beige-Like Anchor Protein), CTLA-4 (Cytotoxic T Cell-Associated Protein 4), PI3KR1 (Phosphoinositide-3-Kinase Regulatory Subunit 1), and PIK3CD (Phosphatidylinosito-4,5-Biphosphate 3-Kinase Catalytic Subunit Delta) [34].