PSEN1 and Alzheimer disease: Consistent with the H4K16ac study, those H3K27ac differential peaks also represented a significant enrichment of AD risk variants, including genetic regions involved in AD neuropathology such as APP, PSEN1, PSEN2, and MAPT. With a sample size of 669 cases from the ROSMAP cohort, Klein et al. conducted ChIP-seq for H3K9ac, another histone mark for transcriptionally active open chromatin, in the dorsolateral prefrontal cortex of control and AD individuals.