Other studies that identified changes in KLK8 expression levels to be linked with recurrent depression were performed in a younger population aged 18–67 (mean = 47.64 years, sd = 11.16)89 than MADT and LSADT cohorts, but more research is required across even younger age groups to assess if findings from this study apply to the general population. This evidence concerns the gene KLK8 and depressive symptom measurement.