SLC6A4 and depressive symptom measurement: In a series of recent reports, we have proposed that alterations in membrane protein clustering in peripheral lymphocytes may represent a novel biomarker of depression: We observed that the pattern of clustering of both the serotonin transporter (SERT) and serotonin 2A receptor on the plasma membrane of lymphocytes from naïve depression patients identified two subpopulations of patients that showed a different therapeutic response after 8 weeks of conventional antidepressant treatment [4–7].