Indeed, by investigating human (MCF-7 and MDA-MB-231) and mouse (4T1) mammary carcinoma cell lines expressing low to undetectable levels of CB1 and CB2, McKallip et al. found that these cells were not only resistant to THC-induced cytotoxicity, but THC treatment led to elevated 4T1 tumor growth and metastasis due to CB2-mediated inhibition of the antitumor immune response (140). This evidence concerns the gene CNR2 and breast carcinoma.