Hereditary, biallelic inactivating mutations of the GCGR gene is clinically characterized by hyperglucagonemia without glucagonoma, hyperaminoacidemia, transformation of hyperplastic islets into glucagon producing microadenomas and PNET, also known as Mahvash disease [17–20,33–35]. The gene discussed is GCG; the disease is GCGR-related hyperglucagonemia.