MPO and breast cancer: The potential for internal comparator controls to impact assessment of HER2 status was not addressed by previous studies but has concerned us since we began characterizing this alteration.2,3,25 In 2 early investigations of HER2 amplification using the same 345 breast cancers, we used myeloperoxidase gene (MPO) as the internal control (HER2-to-MPO ratio)3 whereas another group used TP53 (HER-to-TP53 ratio)42 for assessment of HER2 status.