NRG1 and plexiform neurofibroma: Further, when we overexpressed an erbB3/erbB4-specific ligand [the neuregulin-1 (NRG1) isoform glial growth factor-β3 (GGFβ3)] in the Schwann cells of transgenic mice (P0-GGFβ3 mice), these animals developed neurofibromas [18] that transformed into MPNSTs in vivo [18, 19] with genomic abnormalities analogous to those seen in human MPNSTs [18].