Until very recently, the majority of hereditary pheochromocytomas were related to the MEN 2 and the VHL. In rare instances, hereditary pheochromocytoma was reported in patients with NF1. In addition, nonsyndromic hereditary pheochromocytomas have been reported. Recently, three more genes (SDHD, SDHB, and SDHC) which are all related subunits of the mitochondrial complex II have been identified to cause susceptibility to pheochromocytoma and/or paraganglioma. Hence, mutation analysis of VHL, RET, SDHB, and SDHD is generally recommended in patients with pheochromocytoma regardless of their family history or other features suggestive for a hereditary form. Mutation analysis should start with VHL and RET. However, in the presence of extra-adrenal pheochromocytoma, it may be more useful to screen for VHL, SDHD and SDHB mutations. It is of interest that various different genes can lead to one type of tumor formation. A common pathway (i.e. oxygen sensing) has been shown for VHL and SDHX. However, although several genes that are involved in the pathogenesis of hereditary pheochromocytoma are known, the precise molecular steps in tumorigenesis are widely unknown. In addition, recent data in MEN 2 pheochromocytomas point to a 'second hit' mechanism as a trigger for tumor formation. The molecular pathogenesis of sporadic pheochromocytomas remains obscure [114].