 Borderline personality disorder BPD is a complex psychiatric disease of uncertain aetiology and pathogenesis. A key mechanism of disease susceptibility and treatment response could be epigenetic changes in DNA methylation patterns. However, no study has yet demonstrated that psychotherapy can exert its therapeutic effect through epigenetic mechanisms. The main aim of this study is to analyze the promoter methylation pattern of genes considered to be related to the development and psychopathology of BPD, in particular the brain-derived neurotrophic factor BDNF and glucocorticoid receptor genes, and the effects of mentalization based treatment MBT on changes. Associations to changes in BDNF serum levels and salivary cortisol levels, as well as key components of BPD aetiology and core treatment targets in MBT, will also be investigated. Should epigenetic mechanisms have importance for BPD pathology and effects of treatment, there is potential use of DNA methylation patterns as valid biomarker measures of diagnosis, prognosis, and treatment response. 