Mutations in Alsin are associated with chronic juvenile amyotrophic lateral sclerosis (ALS2), juvenile primary lateral sclerosis and infantile-onset ascending spastic paralysis. The primary pathology and pathogenic mechanism of the disease remain largely unknown. Here we show that alsin-deficient mice have motor impairment and degenerative pathology in the distal corticospinal tracts without apparent motor neuron pathology. Our data suggest that ALS2 is predominantly a distal axonopathy, rather than a neuronopathy in the central nervous system of the mouse model, implying that alsin plays an important role in maintaining the integrity of the corticospinal axons.