Figure 1. The Knudson two-hit model.Diploid cells have two copies of each gene. A tumor suppressor gene, such as the von Hippel–Lindau gene, is consistent with the Knudson model. In the hereditary form of cancer, the patient inherits a mutation of one copy of the gene. In the tumor, such as a pheochromocytoma in a patient with von Hippel–Lindau disease, the second copy is inactivated by a mechanism such as mutation or deletion.