Immunohistochemical study for (a, b) seromucinous tumor, (c, d) clear cell carcinoma, and (e, f) polypoid endometriosis. Estrogen receptor: (a), (c), and (e). Napsin A: (b), (d), and (f). Seromucinous tumor cells and endometriosis cells are positive for estrogen receptor (a, e) but negative for Napsin A (b, f). Clear cell carcinoma cells are positive for Napsin A (d) but negative for estrogen receptor (c).