MAP2K1 and plexiform neurofibroma: Tumors develop after somatic loss of the wild type (WT) NF1 allele and typically show hyperactive MAPK signaling.4 Preclinical and clinical data demonstrate that the MEK1/2 inhibitor selumetinib reduces the size and growth potential of NF1-associated plexiform neurofibromas and low-grade glioma, and selumetinib was recently FDA approved for children with symptomatic inoperable plexiform neurofibroma.5–8 However, the ability to assess drug concentration and effect in NF1-relevant tissues has been a challenge.