Two forms of neurofibromatosis, type 1 (NF1) and type 2 (NF2) are connected with 
genes localized on chromosomes 17 and 22, respectively. The genes that are 
inactivated in neurofibromatosis code for the proteins neurofibromine and 
merline, respectively. Since inactivation leads to neoplasia, they are called 
tumour suppressor genes. Neurofibromine shows resemblances to proteins that 
serve to inactivate oncogenes. Merline has a relationship with proteins that 
connect the cytoskeleton and the cell membrane. The precise function of the 
proteins is still unknown. The NF1 gene is characterized by extraordinarily high 
sensitivity to mutation; half the NF1 patients have not inherited the disease. 
In the familial form of neurofibromatosis, a mutated gene is inherited and the 
normal allele in the tumour is inactivated, making tumour growth possible. In 
the sporadic form of neurofibromatosis, both normal alleles are inactivated 
locally in the tissue so that a tumour develops in that place.