ALK and plexiform neurofibroma: To test whether increased ALK signaling in the absence of NF1 might play a role in the development of neurofibromas, we used reverse transcription/PCR to detect the presence or absence of ALK mRNA in neurofibroma-derived NF1−/− Schwann cells and NF1+/− fibroblasts, using RNAs kindly provided by Drs. Eric Legius and Eline Beert.