Consequently, NFs and particularly the neurofilament light chain (NfL) subunit recently emerged as promising in-vivo markers of active brain pathology in several neuropsychiatric disorders, including multiple sclerosis, neurodegenerative diseases, traumatic brain injuries, and depression (Barro et al. 2018; Khalil et al. 2018; Bavato et al. 2021). Here, NEFL is linked to major depressive disorder.