Though histologically benign, the nine cases of endometriosis that displayed HNF-1β expression included five cases of reactive epithelium and four cases of “atypical endometriosis.” When a series of 40 cases of endometriosis not associated with a primary clear cell ovarian carcinoma were also evaluated, 16 (40%) of the cases displayed HNF-1β positivity. Here, HNF1B is linked to ovarian clear cell cancer.