CHIR-99021 HCl is a potent and selective inhibitor of GSK-3α and GSK-3β kinases. It is widely used in stem cell research to support the maintenance of pluripotency in embryonic stem cells by activating the Wnt signaling pathway. By inhibiting GSK-3, it stabilizes β-catenin, which promotes self-renewal and prevents spontaneous differentiation. It is commonly included in culture media formulations for human and mouse pluripotent stem cells, especially in feeder-free or defined culture systems. CHIR-99021 HCl is also used in reprogramming somatic cells into induced pluripotent stem cells (iPSCs), where it enhances reprogramming efficiency. Additionally, it plays a role in directed differentiation protocols, particularly in cardiac and neural differentiation, due to its ability to modulate key developmental pathways. Its high selectivity and cell permeability make it a valuable tool in regenerative medicine and drug discovery research.