Patients with gain-of-function mutations in PIK3CD, which encodes p110δ, exhibit a primary immunodeficiency, activated PI3Kδ syndrome (APDS), characterized by lymphopenia, lymphoproliferation, and recurrent respiratory infections (Angulo et al., 2013; Coulter et al., 2017; Lucas et al., 2014). The gene discussed is PIK3CD; the disease is respiratory tract infectious disorder.