(R)-JQ1 is a potent, cell-permeable, and selective small-molecule inhibitor of the bromodomain and extra-terminal (BET) family of proteins, specifically targeting the first bromodomain (BD1) of BRD4. It functions by competitively binding to the acetyl-lysine recognition pocket of BET bromodomains, thereby displacing them from chromatin and suppressing the transcription of key oncogenes such as *MYC*. This compound is a widely used chemical probe and a lead compound in oncology research for studying the role of BET proteins in cancer, inflammation, and other diseases. It has demonstrated significant anti-proliferative activity in various cancer cell lines and animal models, particularly in hematological malignancies like acute myeloid leukemia (AML) and multiple myeloma. Beyond its role as a direct inhibitor, (R)-JQ1 serves as a foundational ligand for the development of proteolysis-targeting chimeras (PROTACs) aimed at degrading BET proteins. Due to its high specificity and well-characterized mechanism, (R)-JQ1 is a staple tool in epigenetic and transcriptional regulation research. It is typically handled as a research-grade chemical and is not approved for therapeutic use in humans.