C9orf72 and frontotemporal dementia: C9orf72 HRE carriers and a subset of sporadic FTD/ALS cases show evidence of LINE1 element derepression and retrotransposition, events thought to be related to loss of nuclear TDP-43 (Prudencio et al., 2017; Liu et al., 2019; Tam et al., 2019) and suggested by independent work in Drosophila TDP-43 models (Krug et al., 2017; Chang and Dubnau, 2019).