In the developing central nervous system (CNS), L1CAM is one of the major genes for HSP pathology, which interacts with neuropilin-1(Nrp1) followed by Plexin-A to form Semaphorin 3A (Sema 3A), required for steering away from the corticospinal neurons from the medullary or midline spinal cord. The gene discussed is NRP1; the disease is hereditary spastic paraplegia.