Causal genes in rare neurodegenerative diseases have been linked to neurofilaments, with mutations in the NfL gene causing subforms of Charcot‐Marie‐Tooth Disease (CMT) (Mersiyanova et al. 2000; Higuchi and Takashima 2022; van Asperen et al. 2024), hereditary spastic paraplegia (HSP) (Mul et al. 2020), predisposing to amyotrophic lateral sclerosis (ALS) (van Asperen et al. 2024), spinal muscular atrophy (Ando et al. 2022) and Parkinson disease (PD) (Lavedan et al. 2002). The gene discussed is NEFL; the disease is proximal spinal muscular atrophy.