Immunohistochemical localization of ALDH1 in malignant serous ovarian tumors. ALDH1 expression was heterogeneous in malignant tumors. [A-B] A well differentiated tumor showing ALDH1 expression in numerous cells of epithelium and stroma with varying staining intensities (black arrows) or no staining (white arrow). Note: the nuclei are small, regular and lack prominent nucleoli, which is characteristic of a low grade tumor. [C-D] A poorly differentiated tumor showing absence of ALDH1 expression in tumor epithelial cells (white arrows). A few adjacent tumor stromal cells (black arrow) expressed ALDH1. [E-F] Representative images of ALDH1 staining in a poorly differentiated tumor. Characteristic tumor cells with activated nucleus (white arrow) show no ALDH1 expression, while adjacent stromal tissue contained few ALDH1+ cells (black arrow). Sections were counterstained with hematoxylin. (Original magnifications: ×100, ×400, ×100, ×400, ×100 and ×400 respectively).