Via various receptors 5-HT is thought to function as a growth factor for different malignancies in situ and/or for associated cell lines , e.g., pulmonary carcinoma, prostate carcinoma, pancreatic carcinoid, bladder cancer, glioma and breast cancer [4,5,6], while SERT influences the development of colorectal tumors [7] and breast cancer [8,9]. Here, HTR5A is linked to breast carcinoma.