SDHD and pheochromocytoma/paraganglioma syndrome 1: The genetic basis for hereditary PGL syndrome type 1 (PGL1) was discovered by Baysal et al. (2000) by combining knowledge that hypoxia increases the risk of carotid body PGLs with the presence of a hypoxia-responsive gene encoding succinate dehydrogenase subunit D (SDHD) within a region at chromosome 11q23 linked in family studies to hereditary head and neck PGLs (HNPGLs).