HRH4 and breast cancer: On the other hand, in both in vitro studies as well as in vivo studies performed in immunodeficient hosts, where the role of the immune system in the response to anticancer treatments could not be evaluated, the treatment with H4R agonists showed very promising outcomes.18–21,40,53,54 In line with this, the results indicate that elevated HRH4 gene expression was associated with improved relapse-free survival outcomes in all breast cancer patients.