PIK3R1 and immunodeficiency due to a classical component pathway complement deficiency: In this report, we provide the first description of C1q deficiency occurring in association with genetically confirmed APDS2 caused by a previously well-described de novo splice mutation in PIK3R1, c.1425 + 1G > A, which is a mutation hot spot that accounts for 79% of the 64 patients with APDS2 reported to date (8).