ARID1A and endometrial endometrioid adenocarcinoma: TCGA cases exhibiting C-terminal CCND1 mutations had frequent co-occurring mutations of genes implicated in the pathogenesis of endometrial endometrioid adenocarcinomas, as the majority of CCND1-mutated cases exhibited mutations of ARID1A and members of the PI3K signaling pathway, PTEN and PIK3CA. Less frequent co-occurring mutations were seen in other genes involved in endometrial carcinogenesis such as KRAS, CTNNB1, ARID5B and PMS2 in a minority of CCND1-mutated cases.