Results presented herein (i) show that ErbB2-positive invasive human breast tumors express CB2 receptors, (ii) demonstrate that Δ9-tetrahydrocannabinol (THC) and the non-psychotropic CB2 cannabinoid receptor agonist JWH-133 significantly reduce tumor progression in a clinically relevant model of ErbB2-positive metastatic breast cancer, and (iii) shed light on the mechanism of cannabinoid antitumoral action in vivo. The gene discussed is ERBB2; the disease is breast neoplasm.