Abstract An 11 year old boy presented with a three month history of progressive bilateral lower extremity weakness associated with recent intermittent incontinence. Spine MRI showed a right-sided T11-T12 paraspinal mass extending through the neural foramina into the epidural space and causing severe spinal cord compression. Skin showed a large macular lightly-hyperpigmented café au lait spot with irregular borders along the T10-T12 dermatome extending from the spine to approximately the anterior-axillary line. He was suspected to have segmental neurofibromatosis type 1 (NF1) as no additional clinical findings, radiographic features or family history of NF1 were identified. Patient underwent T10-12 laminectomy for resection of the epidural tumor component. Post-operative MRI showed resolution of the mass effect on the thecal sac and cord, with expected tumor residual lateral to the neural foramen. His residual spinal tumor and mild scoliosis remained stable over the two years of follow up to date. Pathological and molecular analysis of the resected tumor revealed a neurofibroma harboring an activating KRAS c.35G>A, p.Gly12Asp (KRAS-G12D) pathogenic variant at 27% variant allele frequency. Melanocytes cultured from two hyperpigmented skin biopsies showed the same KRAS-G12D pathogenic variant. This KRAS-G12D pathogenic variant was not found in leukocytes, indicating a post-zygotic origin. No NF1 pathogenic variant was identified in tumor tissue, melanocytes or leukocytes. The clinical findings were consistent with a mosaic KRASopathy due to a post zygotic KRAS-G12D pathogenic variant. The presence of the KRAS variant in the spinal neurofibroma and overlaying café au lait spot without an NF1 etiology in associated tissues demonstrates overlapping variability of presentations of RAS-MAPK pathway disorders. This case highlights the need for full clinical and genetic evaluation of patients presenting with segmental neurocutaneous disorders.