BDNF and major depressive disorder: Moreover, given that chromatin modifications are associated with stress-induced illness, memory impairments, depression, and different phases of BP [393, 394, 402–406, 415, 417, 418], it seems likely that epigenetic profiles in the peripheral blood, particularly in BDNF, could be used as a biomarker for those who are susceptible to MDs, as well as for tracking therapeutic response following PA interventions in persons with MDs [30].