A similar clinical phenotype of agammaglobulinemia or hypogammaglobulinemia is observed in patients with hemizygous variants in BTK, biallelic variants in BLNK, heterozygous gain-of-function (GOF) PIK3CD, or loss-of-function (LOF) PIK3R1 variants, which all encode proteins involved in BCR signaling (38, 39, 44, 45). This evidence concerns the gene BTK and agammaglobulinemia.