The clusters may be solid or show acinar formations. The mucin is extracellular and acidic or neutral type. An in-situ component is usually absent in mucinous carcinomas of breast. Mucinous carcinoma shows strong cytoplasmic positivity for MUC2 and decreased MUC1 immunoreactivity when compared to ductal carcinoma. Hormone receptors ER and PR are always positive; HER2 is negative. Many cases express WT1. Between 25% to 50% of cases show neuroendocrine differentiation and are positive for NSE and related markers.