Mutations in the PIK3CD and PIK3R1 genes lead to a gain-of-function in APDS, a monogenic condition, as they encode crucial elements of the PI3Kδ pathway. PIK3CD encodes the catalytic subunit of PI3Kδ, p110δ, and PIK3R1 encodes the regulatory subunit p85α [6]. Mutations in these genes lead to hyperactivity of the PI3Kδ signaling pathway, which interferes with regular immune activity [14]. Mutations in PIK3CD, found in APDS1, usually cause amino acid substitutions that increase p110δ enzyme activity, such as the E1021K mutation [15]. The gene discussed is PIK3CD; the disease is activated PI3K-delta syndrome.