SLC6A4 and depressive symptom measurement: However, the analysis of the distribution of SERT cluster size allowed us to differentiate two subpopulations of naïve depression patients that we named D-I and D-II, the D-I subpopulation represented about 3⁄4 of the patients and showed more than 40% of SERT clusters being between 0.05 and 0.010 μm2 (the modal peak of cluster size), while the DII subpopulation showed around 25% of SERT clusters between 0.05 and 0.010 μm2 (Rivera-Baltanas et al., 2012).