Furthermore, Kendler and Flint summarized a number of genes (5HTTP/SLC6A4, APOE, DRD4, GNB3, HTR1A, MTHFR, and SLC6A3) linked to susceptibility to heterogeneous depression by interpreting a large amount of candidate gene literature in 2014 (11). This evidence concerns the gene SLC6A4 and depressive symptom measurement.