In the case of the sibsof a proband, the risk of VHL syndrome to sibs depends upon the genetic statusof the parents: if a parent of a proband is clinically affected or has a disease-causing VHL mutation, the sibs ofthe proband are at 50% risk of inheriting the altered gene; and if neither parent has the disease-causing VHL mutation identified in the proband, the sibs have a small risk of VHL syndrome because ofthe possibility of germline mosaicism in one parent (at present the incidence of mosaicismis not known) [24]. The gene discussed is VHL; the disease is von Hippel-Lindau disease.