<p>Serotonin-pathway genes versus immune signatures across TCGA cancers.</p>

<p>Median Spearman correlation of each of 20 serotonin-pathway genes (the transporter SLC6A4, 14 serotonin receptors, the synthesis enzymes TPH1/TPH2/DDC, and the degradation enzymes MAOA/MAOB) with three immune signatures (T-cell-inflamed GEP, IFN-gamma, and cytolytic score) across 20 TCGA solid-tumor types (1,200 correlations (20 genes × 3 immune signatures × 20 cancers; pooled to 60 gene-by-signature tests in the main text); Benjamini–Hochberg corrected).

SLC6A4 ranked first of 20 by pooled correlation (median rho −0.16; significant-negative in 37 of 60 gene-by-signature tests; significant-positive in only 2). The degradation enzyme MAOB correlated positively and most receptors showed weak or positive associations, indicating that the inverse association with immune signatures tracks specifically with the serotonin-handling transporter rather than the pathway at large. Blue = negative correlation; red = positive.