Furthermore, the role of serotonin in tumor biology in vivo has been elucidated using a genetic model of serotonin deficiency (tph1-/-) in mice, and researchers concluded that serotonin regulated angiogenesis in colorectal cancer allografts by influencing matrix metalloproteinase 12 expression in tumor-infiltrating macrophages, thereby affecting the production of circulating angiostatin [70]. Here, TPH1 is linked to colorectal cancer.