ARID1A and endometrioid adenocarcinoma: While ARID1A mutations are common in endometrioid adenocarcinoma, co-inactivation of ARID1A and 1B, the mutually exclusive subunits of the BAF complex, is required for the formation of dedifferentiated and undifferentiated endometrial carcinoma, which shows undifferentiated, monotonous, occasionally rhabdoid tumour cells on histology, with loss of expression of PAX8 and ER, and clinically aggressive biological behaviour [70,71].