This occurs frequently before or upon the first presentation of HAE-like symptoms (such as unexplained gastrointestinal symptoms) in children with a positive family history of C1-INH-HAE, where the functional and antigenic C1-INH levels are not reliable parameters of C1-INH-HAE before the first year of age, as their reference ranges are significantly lower compared to the adult reference ranges (8, 9, 41). Here, SERPING1 is linked to hereditary angioedema.