Mutations in the AT-rich interactive-domain-containing protein 1A (ARID1A) gene, which encodes BAF250a, a component of the SWI/SNF family of chromatin remodeling complexes, are frequently observed in ovarian clear cell carcinomas, endometrioid carcinomas, pancreatic adenocarcinomas, transitional cell carcinomas, and triple-negative breast cancers [208,209,251]. The gene discussed is ARID1A; the disease is transitional cell carcinoma.