FKBP5 and post-traumatic stress disorder: Using a sub-sample from the recent Phase 3 clinical trial, we conducted a pilot study investigating the methylation of three key PTSD-relevant HPA genes in association with MDMA-assisted therapy treatment response (3); NR3C1, FKBP5, and CRHR1. The gene NR3C1 encodes for a glucocorticoid receptor (GR) which plays a role in the HPA negative feedback loop.