 According to our hypothesis, the severity of a sub-phenotype will correlate with heritable differences in the germline expression of certain genes. As an example, consider a spectrum of individuals affected with the primary sub-phenotype of spinal neurofibroma burden. For most genes, there will be no relationship of expression level to the number of spinal neurofibromas. However, some genes will have a direct or inverse, or other correlation of their level of expression and the number of neurofibromas. Such genes would be considered as candidate modifier genes. The secondary sub-phenotypes will also be analyzed in a similar way. To limit false positives, candidate genes will then be tested for association using the transmissiondisequilibrium test, TDT with the appropriate sub-phenotype. 