Neural-Derived Plasma Exosomal MicroRNAs As Promising Novel Biomarkers for Suicidality and Treatment Outcome in Adolescents

Inclusion Criteria: All participants: 1. Physically healthy 2. willing and able to provide informed consent (if under 18 also parent or guardian consent) MDD participants: 1. A definite diagnosis of DSM-5 2. a Children's Depression Rating Scale-Revised (CDRS-R) score \>=30. Suicidal ideation participants: Columbia Suicide Severity Rating Scale (C-SSRS) score \>=4 rated over the last two weeks. Suicide attempt group: 1\. Participants will have had an attempt in the previous two weeks that is serious enough to require medical attention and shows evidence of at least a medium level of intent on the Suicide Intent Scale. Non-psychiatric controls: 1\. No history of any major mental illness (excluding specific phobia) or substance use disorder. Exclusion Criteria: * Exclusion criteria: 1. Pregnancy or lactation 2. post-partum state (being within 2 months of delivery or miscarriage); 3. homicide risk as determined by clinical interview 4. any of the following DSM-V diagnoses or categories: a) a lifetime history of psychotic disorder; b) alcohol or drug use disorder (except nicotine/caffeine) within the last month; the use of any hallucinogen (except cannabis), including phencyclidine in the last month; c) bipolar disorder; d) pervasive developmental disorder; e) cognitive disorder; f) DSM-5 paranoid, schizoid, or schizotypal personality disorders (PDs) (participants with other PDs will be allowed as long as MDD criteria are met); g) anorexia nervosa. 5. recent myocardial infarction or unstable angina, active neoplasm in the past 6 months, immunosuppressive or corticosteroid therapy within the last month, chemotherapy, and head injury or loss of consciousness in the past 6 months 6. use of hallucinogens (except for cannabis), methamphetamine, or cocaine in the last 2 weeks.