ESR1 and clear cell adenocarcinoma: Immunohistochemical (IHC) staining performed on the larger ovarian mass showed strong nuclear staining for HNF1-beta and PAX-8 (Figure 3c,g), variable intensity cytoplasmic staining with napsin A, and only scattered, weak-to-moderate intensity nuclear staining with estrogen receptor (ER) (Figure 3e), consistent with a clear cell carcinoma.