The research group of Mikko Seppänen is a group of clinicians, geneticists, and basic immunologists dedicated to seek novel genetic primary immunodeficiencies (PID), including novel diseases in the Finnish Disease Heritage. We have a wide net of collaborators in a.o. molecular biology, hematology, rheumatology, dermatology, and gastroenterology. The main focus is currently on PIDs discovered in clinical whole exome or whole genome sequencing in children and adults. Our group has a.o. described an autosomal dominant STAT3 gain-of-function autoimmune PID, an autosomal recessive combined PID leading to severe congenital neutropenia and specific antibody deficiency (HYOU1) and a Behcet-like disease and familial necrotizing cellulitis caused by differently acting mutations in NFKB1. We have also found a 37-fold enriched Finnish founder mutation in AICDA causing autosomal recessive hyper-IgM syndrome type 2. Further discoveries are studied. These studies pave way to understanding the pathogenesis of somewhat more common and difficult to treat diseases like different autoimmune diseases, common variable immunodeficiency, Mb Behcet and necrotizing fasciitis.