Silencing of 5-HT7 (but not of 5-HT1D) receptor (Fig. 2c and d) abrogated 5-HT-induced MDA-MB-231 cell invasion (Fig. 2e), and this was accompanied by a decrease in the expression of matrix metalloproteinase-9 (MMP-9), a positive regulator of cancer cell invasion (Fig. 2c and d). Here, MMP9 is linked to cancer.