A potent and highly selective inhibitor of glycogen synthase kinase-3β (GSK-3β) (IC₅₀ = 6.7 nM). CHIR99021 has been shown to allow for long-term expansion of murine embryonic stem cells in a chemically-defined medium in conjunction with MEK/MAPK inhibitor PD184352 and fibroblast growth factor receptor (FGFR) inhibitor SU5402.