PGR and neoplasm: Thus, a correlation was established between CB2 expression and tumor aggressiveness, since CB2 mRNA levels were higher in ER-/PR- tumors than in ER+/PR+ tumors, as well as in HER2+ tumors than in HER2- tumors, and in tumors with high histological tumors of low malignancy than in histological tumors of low malignancy.