OBJECTIVE: Unhealthy family functioning and Protocadherin 9 ( METHODS: A cohort of 6985 first-year university students in China was established in 2018 and was followed for one year. The blood samples from 4866 of these participants (38.88% males, mean age:18.42) were genotyped. MDD was assessed using the Composite International Diagnostic Interview-3.0 (CIDI-3.0), while family functioning was evaluated with the Family Assessment Device (FAD). Logistic regression was used to examine the interaction of these two risk factors. RESULTS: The one-year prevalence of MDD was 2.4%. Among first-year university students, the AA genotype of rs9540720 ( CONCLUSION: University students carrying the AA genotype of rs9540720(