Those of a. Knudson 1971 postulates that tumour development only occurs if both alleles of the (onco-) gene are inactivated. This applies, for example, to autosomal dominant inherited neurofibromatosis type II. Here, tumour development occurs early in the life of the affected person. In sporadic cases (e.g. sporadic formation of schwannomas) only 1 allele is affected due to a congenital germ cell mutation or a somatic de novo mutation ("first hit"). If a further de novo damage ("second hit") of the remaining, normal (wild-type) NF2 allele occurs in the course of life, its function is inactivated (physiological tunor suppressor function). The result is tumor development of the cells, which lose the suppressive function of the remaining, normal (wild-type) NF2 allele.