SLC6A4 and major depressive disorder: This implies that analysis of membrane protein clustering of SERT and/or 5-HT2A (as the D-I and D-II depression subpopulations have been equally demonstrated by employing any of those markers as we previously have shown [11, 12]), identifies two subpopulations of naïve depression patients, one of which (the D-II group) shows a good improvement in anhedonia symptoms upon pharmacological treatment, while the other group (i.e., D-I) fails to show any response.