According to the study of a large number of malignant primary tumor tissues from pheochromocytomas and paragangliomas, PD-L2 expression but not PD-L1 expression is significantly associated with stronger hypoxia-driven HIF-1α and carbonic anhydrase 9 (CAIX) (62). The gene discussed is CA9; the disease is hereditary pheochromocytoma-paraganglioma.