MT-CYB and major depressive disorder: However rare homoplasmic mutations with possible functional consequences were reveled in major depression cases, in the ATP synthase 8 (ATP8), ATP synthase 6 (ATP6), ND5 and cytochrome b (CYTB) genes, while another subject with depression demonstrated subthreshold heteroplasmy rate at a variant in the displacement loop (D-loop) part of mtDNA (142).