Reduced or absent neurofibromin activity has been associated with the permanent activation of Ras (high Ras-GTP levels), leading to a constant stimulation of the Raf–MEK–ERK signalling cascade, which in turn promotes cell proliferation and tumour growth (Figure 3) [265,271,272,273,274,275,276,277,278]. Here, NF1 is linked to neoplasm.