The severity and physical signs of NF1 can vary widely from patient to patient, even among people from the same family. Scientists don't know exactly why this happens, but they predict that genetics has a lot to do with it.

It is likely that each patient's unique genetic makeup influences the severity of his or her NF1. That is, one or more additional genes other than the NF1 gene might also play a role. Scientists call these "modifier genes," and they could be any of the thousands of genes in the human genome.

How do modifier genes work? Proteins encoded by NF1 modifier genes might work in the same biological pathways as neurofibromin, so they too could affect how well these pathways work. Variations in the DNA sequence of a modifier gene may alter its function enough to influence the severity of NF1 in an individual.

To date, scientists have not found any strong candidate NF1 modifier genes.