Double immunohistochemical staining for PR/HER2. To validate the lab staining method, commercial breast cancer tissue microarray (TMA) slides BR701 (US Biomax Inc.) were used. D1, TMA core with HER2+ and PR- staining. D2, HER2- and PR+ staining. G1, HER2+ and PR+ staining. D9, PR+ and HER2 weak-positive (borderline) staining. PR/HER2 double stains were comparable to standard single stains for HER2 and PR, although the PR signal in the double staining was somewhat weaker (original magnification: ×200).