HRH4 and breast carcinoma: It was reported that histamine and histamine H4 receptor (H4R) agonists inhibit proliferation of two human breast cancer cell lines in vitro and in vivo, augmenting tumoral apoptosis.16–21 Similar anti-tumor effects of H4R agonists were observed in three different human melanoma cell lines22 and in cholangiocarcinoma experimental models.23