Notably, selective antagonists of TPH1, SERT, and a subset of 5-HTRs (5-HT1B, 5-HT1D, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT5A, and 5-HT6) all reduced tumor cell viability and tumorsphere-forming cell frequency in all the breast tumor cell lines. This evidence concerns the gene SLC6A4 and neoplasm.