The purpose of this study is to identify the variations in gut microbiota compositions between two subtypes of major depressive disorder.

Objective: To identify the variations in gut microbiota compositions between two subtypes of major depressive disorder (MDD), namely those with and without REM sleep behavior disorder (RBD). To correlate the microbiota composition with depressive severity and prodromal markers of α-synucleinopathy.

Design: Case-control study Setting: Established clinical cohorts consisting of patients with comorbid MDD+RBD and MDD only and a group of sexand age-matched healthy control Participants: Patients with comorbid MDD+RBD (n=55), patients with MDD only (n=55), and healthy control (n=55).

Main outcome measures: The abundance and composition of gut microbiota, severity of depressive symptoms, and prodromal markers of α-synucleinopathy.

Inclusion Criteria:

1. Age-and sex-matched with MDD+RBD subjects;
2. Free of psychiatric disorders based on M.I.N.I.;
3. Without a personal history or a family history of RBD or neurodegenerative diseases (i.e. dementia and PD)
4. Free of RBD symptoms or RWSA by vPSG

Exclusion Criteria:

1. Presence of narcolepsy and other neurodegenerative diseases that may give rise to RBD and RWSA;
2. Severe psychiatric illness that could not permit a valid written informed consent or otherwise is not suitable for participating in a study;
3. Use of antibiotics within one month and the use of probiotics within three days prior to sample collection;
4. Pre-existing or post-operation of gastrointestinal diseases (e.g. inflammatory bowel disease, gastrointestinal cancer).

The subjects of the current study would be recruited from our established cohort of patients with MDD (with and without RBD). Three groups of subjects are included in the current study, namely healthy control, patients with MDD only, and patients with MDD+RBD.

Data will be handled and stored under Department of Psychiatry and PI will be responsible for the safekeeping.