This study is dedicated to help identify biomarkers for depression and suicide. The purpose of the study is to better understand these links to improve medical and psychiatric care in the future. This research is also to test the effects of standard treatment of depression on improvement in depressive and suicidal behavior and on biomarkers (e.g. miRNA) for these disorders.

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.