Breast myoepithelial markers: Problems and pitfalls for the practicing pathologist

Immunohistochemical stains for myoepithelial cell (MEC) markers are commonly used in breast pathology to help distinguish benign and in situ lesions from invasive carcinomas. Benign and in situ lesions typically demonstrate an associated MEC layer, whereas invasive carcinomas typically lack associated MECs. However, there are some benign lesions and in situ carcinomas in which a MEC layer is partially or completely absent, and there are malignant lesions that contain a component of MECs. An understanding of these exceptions is critical to ensure the correct diagnosis. This review will focus on benign and in situ breast lesions in which associated MECs are reduced or entirely lacking, and malignant breast lesions that contain a population of MECs.