C9orf72 and amyotrophic lateral sclerosis: Understanding the genetic landscape of normal C9orf72 - that is, genes that are normally co-expressed with non-expanded C9orf72 - may clarify why certain brain regions are selectively targeted in ALS or FTD (henceforth, ALS/FTD), why some patients may be more likely to develop either ALS/FTD, or both; and which cell-type populations and biological mechanisms are involved.