Background: Breast cancer is the most common cancer in Myanmar women. Revealing the hormonal receptor
status, human epidermal growth factor receptor 2 (HER2) and Ki-67 expression is useful for estimating patient
prognosis as well as determination of treatment strategy. However, immunohistochemical features and classification of
molecular subtypes in breast cancers from Myanmar remain unknown. Methods: The clinicopathological features of
91 breast cancers from Myanmar women were examined. Immunohistochemistry was performed on tissue specimens
with antibodies to estrogen receptor (ER), progesterone receptor (PgR), HER2, Ki-67, cytokeratin (CK)5/6 and CK14.
Immunohistochemistry-based molecular subtyping was conducted. Results: Breast cancers in Myanmar women were
relatively large, high grade with frequent metastatic lymph nodes. Of the 91 patients, tumors with ER positive, PgR
positive, and HER2 positive were 57.1%, 37.4%, and 28.6%, respectively. The most prevalent subtype was luminal B
(HER2-) (39.6%), followed by HER2 (22.0%), triple negative (TN)-basal-like (12.1%), luminal A (11.0%), TN-null
(8.8%) and luminal B (HER2+) (6.6%). The mean Ki-67 expression of 91 cases was 33.9% (33.9% ± 19.2%) and the
median was 28% (range; 4%-90%). The mean Ki-67 expression of luminal A, luminal B, HER2 and TN-basal-like/
null was 7%, 30%, 40%, and 57%/43%, respectively. A higher Ki-67 expression significantly correlated with a higher
grade, larger size and higher stage of malignancy. Conclusions: We, for the first time, investigated the histopathological
features of breast cancers from Myanmar women. Myanmar breast cancers appeared to be aggressive in nature, as
evidenced by high frequency of poor-prognosis subtypes with high level of Ki-67 expression.