When this algorithm is applied to discovery cohort specimens including SDHB-null, SDHD-null, and VHL-null PPGL tumors from various anatomical locations, as well as normal adrenal medulla specimens, the hierarchical clustering algorithm generally separates normal adrenal samples, VHL-null pheochromocytomas, and SDHD-null head and neck paragangliomas away from the remaining SDHB-null and SDHD-null PPGL tumors (Fig. 3c). The gene discussed is SDHD; the disease is pheochromocytoma.