C9orf72 and amyotrophic lateral sclerosis: A previous study reported that the majority of layer 5 (L5) cortical neurons exhibiting C9orf72 promoter activity were CTIP2-positive32, suggesting that sub-cerebral projection neurons, which include the corticospinal neurons (CSNs) that degenerate in ALS, express C9orf72. To further test this hypothesis, we compared the distribution of C9orf72 promoter activity in retrogradely labelled CSNs and in unlabeled NeuN-positive neurons intermingled within L5 of motor cortex (Fig. 3a,b).