neurofibroma where the gene dosen't show it self

i was tested for my son on NF useing the gene test. i was told by the doc that i was in a 5% group where the neurofibroma  gene is not showing it self and this was the second time i had it done the first time it showed i had type 1. i was just wondering if anyone knew howthat can be because the doctor here is confused as much as i am on how it happend. i was told it was rare for something like this outta of 100 people only 5% have what i have. can anyone help me out???

[Jordanna Joaquina, MS, CGC]

Neurofibromatosis type 1 (also known as NF type 1 or von Recklinghausen disease) is a genetic condition characterized by changes in skin coloring (pigmentation) and the growth of tumors along nerves in the skin, brain, and other parts of the body. The signs and symptoms vary greatly even among affected members of the same family.

About 1 in 3,000 to 1 in 4,000 people worldwide are affected with NF type 1. It is one of the most common inherited disorders.

NF type 1 is caused by mutations in a gene called NF1, which is located at 17q11.2. We all have two copies of the NF1 gene - one from each parent. In NF type 1, having a mutation in a single gene copy is sufficient to cause the disease. This is called autosomal dominant inheritance. A person with a mutation has a 50% chance with each pregnancy to have an affected child and a 50% chance to have a child that doesn't inherit the mutation and therefore, does not have NF.

We recommend that you meet with a medical geneticist who can review your personal history, your son's history, your family history and all test results. A medical geneticist is a doctor who specializes in genetic conditions such as NF. You can find a medical geneticist at the American College of Medical Genetics website. We wish you the best.