NF1 and plexiform neurofibroma: We found that STAT3 and Ref-1 are expressed in human Schwann cells, MPNST tumours and cell lines, as well as in MPNST that spontaneously develops from existing plexiform and atypical neurofibroma precursor lesions that harbour conditional ablation of Nf1 and Arf in neural crest-derived Schwann cells.