 We hypothesize that normal germline variation in gene expression accounts, in part, for variation in the clinical severity and phenotype of the monogenic disorder neurofibromatosis type 1 NF1. The phenotype of NF1 is constituted by a variety of quantifiable features; we term these features sub-phenotypes . Our main focus is on sub-phenotypes with published evidence of variation in expression from an inherited component. These include our primary sub-phenotype of spinal neurofibroma burden quantified by MRI of spinal neurofibromas and the secondary sub-phenotypes of dermal neurofibroma burden, head circumference, number of Lisch nodules, scoliosis, history of plexiform neurofibromas, and height. Other sub-phenotypes, as collected by a routine history and physical, will also be evaluated. 