While two-thirds of our clear-cell cases stained with HNF1β, 31.6% of serous carcinomas and 13.7% of endometrioid carcinomas expressed HNF1β, making HNF1β staining an unreliable marker in the clear cell versus serous/endometrioid differential. The gene discussed is HNF1B; the disease is serous adenocarcinoma.