Since aromatase is the key enzyme in the estrogen synthesis and is expressed in both breast carcinoma cells, surrounding stromal cells as well as endothelial and benign breast cells [27,34], many attempts have been made to evaluate the importance of aromatase expression in the different cell types in the breast for local estrogen production and for utility as prognostic and predictive marker. Here, CYP19A1 is linked to breast carcinoma.