C9orf72 and amyotrophic lateral sclerosis: Through quantitative comparisons among different types of neurons and glial cells labelled with retrograde neuronal tracers and cell type-specific markers, we demonstrate that mouse C9orf72 promoter activity, although widespread throughout the brain and spinal cord, is specifically enriched in corticospinal and spinal motor neurons and in oligodendrocytes, subsets of cells known to undergo degeneration in ALS, in regions affected by ALS.