We subsequently established a connection between serotonin and BTIC by demonstrating that mouse [8] and human [9] breast tumor cells synthesize serotonin and that antagonists of SERT inhibit BTIC activity using multiple orthogonal assays and synergize with chemotherapy to inhibit the growth of breast tumor allografts and xenografts in vivo. The gene discussed is SLC6A4; the disease is breast neoplasm.