Abstract Background: Myoepithelial cells (MECs) can be visualized easily in normal breast ducts and acini, but when these struc-tures dilate or are compressed, it is almost impossible to identify them on hematoxylin and eosin (H&E) stained sections that's why immunohistochemical markers are used to visualize MECs. Many MECs markers are commonly used. DOG1 was initially known as a marker for gastrointestinal stromal tumors (GISTs) and was not studied before as breast MECs marker. Aim of Study: This study aimingat assessing the immuno-histochemical expression of DOG1 in reactive, benign, insitu, and malignant breast lesions to evaluate its usefulness as a novel myoepithelial marker. Material and Methods: The cohort consisted of 90 cases: Thirty benign lesions, 30 invasive carcinomas (infiltrating duct carcinoma NOS, and infiltrating lobular carcinoma NOS), and 30 noninvasive breast carcinoma (DCIS), as formalin fixed paraffin embedded tissue blocks from archives of Pa-thology department, Kasr Al-Ainy Faculty of Medicine, Cairo University and Nasr City Health Insurance Hospital, Cairo, inthe period from January 2013 to January 2020. All cases were stained for P63 and SMA as a gold standard comparison. Results: Were interpreted using H-score (semi quantitative assessment of both the intensity of staining and the percentage of positive cells). Benign cases showed 100% positivity in MECs, carcinoma in situ (DCIS) staining was positive in 100% of cases, however intensity and percentage were variable. All invasive lesions showed no staining. Conclusion: DOG1 is believed to bea useful marker of breast MECs with excellent sensitivity and specificity, and by adding DOG1 to the MECs identification immunohisto-chemical panel, this will provide more information when diagnosing is not simple.