The aim of our study was threefold: first, to test for aberrant methylation in BPD of functionally important, a priori specified regions (CGI-1, CGI-2, EH1) within the OPRK1 promoter, which could result in different gene activity; further, within a 10 kb analysis window in the promoter, to identify differentially methylated CG sites between BPD and HC; second, to exploratorily analyze correlations between CG methylation rates and BPD severity; and third, to investigate the effects of CT on methylation rates. Here, OPRK1 is linked to Borderline personality disorder.