HRH1 and colonic neoplasm: In the three cell lines analyzed in detail in this study, histamine did not demonstrate any such effect, not even in HT-29 cells, that express a functional H1R. Thus, we conclude that the possible impact of histamine on colon tumor cell proliferation is not mediated via H1R. Indeed, regulation of cell proliferation by histamine may be mediated exclusively by H4R and, thus, could not be detected in the cell lines analyzed.