DAO and major depressive disorder: However, dysfunction of SRR, the d-serine degrading d-amino acid oxidase (DAO), and the main d-serine transporter alanine-serine-cysteine transporter 1 (Asc-1) are implicated in the development of schizophrenia, Alzheimer's disease, and depression [17], [18], [19], [20], [21].