GPR68 and breast cancer: Only weak GPR68 immunosignals could be observed in the colon carcinoma cell line HT-29, the hepatocellular carcinoma cell line Hep-3B, the prostate cancer cell lines PC-3 and LNCaP, the breast cancer cell lines MCF-7 and MDA-MB-231, the cervical cancer cell line ME-180, and the epidermoid carcinoma cell line A431.