The hallmark of neurofibromatosis type 1 (NF1) are multiple dermal neurofibromas. They show high inter- and intrafamilial variability for which the influence of modifying genes is discussed. NF1 patients presenting microdeletions spanning NF1 and several contiguous genes have an earlier onset and higher number of dermal neurofibromas than classical NF1 patients, pointing to one of the deleted genes as modifier. Expression analysis of 13 genes of the microdeletion region in dermal neurofibromas and other tissues revealed four candidates for the modification of neurofibroma formation: CENTA2, RAB11FIP4, C17orf79, and UTP6.