PIK3CD and activated PI3K-delta syndrome: Activated PI3K delta (PI3Kδ) syndrome (APDS) is a rare monogenic PID, caused by heterozygous mutations in either the PIK3CD or PIK3R1 genes, encoding the p110δ catalytic subunit or the p85α regulatory subunit of PI3Kδ, respectively (Jamee et al., 2019).