SLC6A4 and neoplasm: In Sert-WT CD8 T cells, antigen stimulation significantly upregulated the expression of Sert, Tph1, and Maoa genes, which encode the key proteins regulating serotonin transport, synthesis, and degradation, respectively (Figures 5C and S5H).18 Consistently, these key serotonergic genes were dramatically upregulated in antigen-experienced (PD-1hi) tumor-infiltrating CD8 T cells in vivo (Figures 1A and S5I–S5L).