Balanced translocations, each involving chromosome 17q11.2, have been described in two patients with von Recklinghausen neurofibromatosis (NF1). To better localize the end points of these translocation events, and the NF1 gene (NF1) itself, human cosmids were isolated and mapped in the immediate vicinity of NF1. One cosmid probe, c11-1F10, demonstrated that both translocation breakpoints, and presumably NF1, are contained within a 600-kilobase Nru I fragment.