C9orf72 and amyotrophic lateral sclerosis: To explore whether H3K27me3 is differentially marked in ALS PBMCs, we performed a genome-wide analysis of H3K27me3 occupancy using ChIP-seq in a comparison with 3 sample groups (i.e., healthy control and two groups of ALS samples carrying either C9orf72 or SOD1 mutations) (Fig. 1a, Supplementary Table S1).