All screening strategies, second tier tests, and combinations of both are able to identify patients with SCID (N = 1), heterozygous FOXN1 variant (N = 1), Noonan syndrome (N = 1), and RECQL4 (N = 1), while not identifying or “missing” one 22q11.2 deletion syndrome patient and one patient with idiopathic T-lymphocytopenia. Here, FOXN1 is linked to Noonan syndrome.