Immunohistochemical localization of ALDH1 in benign serous ovarian tumors. [A] Uninvolved regions adjacent to benign tumors have a similar staining pattern as normal ovary. [B] Intense staining was observed in stromal (S) and surface epithelial (SE) cells of benign tumors. The inset shows a primary antibody control (anti-ALDH1 omitted). [C] The ALDH1 staining pattern surrounding neo-angiogenic blood vessels (arrow) in a benign tumor ovary differs from normal ovary. [D] Discontinuous pattern of ALDH1 expression was observed along the surface epithelium of benign tumor projections. [E] ALDH1 staining was predominantly expressed in surface epithelial cells of serous papillary projections with less apparent staining of stroma compared to uninvolved areas adjacent to benign tumors. [F] A representative example from the same tissue sample as in E with little or no ALDH1 expression. Sections were counterstained with hematoxylin. (Original magnifications: ×200, ×400, ×400, ×200, ×400 and ×400 respectively).