Importantly, the majority of TCGA cases exhibiting c-terminal CCND1 mutations had frequent co-occurring mutations of ARID1A and members of the PI3K signaling pathway, PTEN and PIK3CA, suggesting that alterations these molecular pathways are also required in addition to oncogenic nuclear cyclin D1 activation in a subset of endometrial cancers. The gene discussed is ARID1A; the disease is endometrial cancer.