Except for somatotroph tumors, which is the major source of GH excess, PIT-lineage-derived mammosomatotroph tumors, mature plurihormonal PIT1-lineage tumors, immature PIT1-lineage tumors, acidophil stem cell tumors, and mixed somatotroph-lactotroph tumors also preserve the ability to secrete excess GH. The gene discussed is GH1; the disease is growth hormone-producing pituitary gland neoplasm.