Digital Atlas of Breast Pathology by Meenakshi Singh, MD © - Department of Pathology, Stony Brook University Medical Center

Invasive mammary carcinoma, CHG 2 of 3, PR positive, high power
Moderately differentiated. Immunohistochemical staining for progesterone receptor also results in a dark nuclear stain if progesterone receptors are present. When more than 20% of the tumor nuclei show a dark nuclear staining pattern, the tumor is considered positive for progesterone receptors.