ARID1B and endometrioid adenocarcinoma: Three of the ARID1A-mutated tumours also harboured frameshift and nonsense alterations in ARID1B. Alterations in ARID1A have been reported in other gynaecological cancers, including a high proportion of ovarian clear cell carcinoma (57%)18, endometrioid carcinoma of the ovary29 and endometrial tumours24, as well as a range of other tumour types such as neuroblastoma, colorectal, gastric, prostate and breast cancers23, 30.