To estimate the potential contribution of autocrine 5-HT to cell invasion, we knocked down TPH1 expression using TPH1 siRNA, and it was found that this inhibited the invasion of MDA-MB-231 cells in the presence of serum (Fig. 2f), suggesting that the important autocrine function of 5-HT during MDA-MB-231 cancer cell invasion. Here, TPH1 is linked to cancer.