Histamine H4 receptor (H4R) is also a GPCR [20,24,25] and is mainly expressed in immunocompetent cells, including mast cells, eosinophils, neutrophils, monocytes, dendritic cells (DCs) and T cells; as well as in intestinal epithelia, spleen, lung, synovial tissue, the central nervous system (CNS), sensory neurons, and cancer cells [21,24,26]. This evidence concerns the gene HRH4 and cancer.