Cancers develop changes in their genes, as well as changes in parts of the cell that control genes. BCOR is a gene that regulates cells by controlling genes, and is changed in a wide range of cancers affecting the blood and organs. By studying a group of more than 20,000 patients with cancer, we saw that the type of BCOR mutation found in a cancer depends on the tissue in which the cancer arises, suggesting that BCOR may have a range of different roles. In patients with endometrial cancer, BCOR mutations are common, and affect a specific part of the gene. The first goal of our study is to describe the clinical impact of BCOR mutations in a large group of patients with endometrial carcinoma. The second goal is to understand how BCOR mutations affect the function and contribute to cancer in cells from different tissues.