Because if you want to test for inheritance of a families predisposition, you need to know which gene is affected in that family. I. This is a common, fully penetrant, autosomal dominant disorder. Incidence 1 in 3000 4000. Half of the cases are caused by new mutations. There is a wide range of manifestations associated with disease alleles.
Some patients have only caf au lait spots.
Others patients have neurobromas. Somatic mutations may also play a role.