CHIR99021 is an aminopyrimidine derivative that is an extremely potent glycogen synthase kinase (GSK) 3 inhibitor, inhibiting both GSK3β (IC₅₀ = 6.7 nM) and GSK3α (IC₅₀ = 10 nM) (Ring et al.). GSK3 is a serine/threonine kinase that is a key inhibitor of the WNT pathway; therefore CHIR99021 functions as a WNT activator. It shows little activity against a large panel of kinases including CDK2 and other serine/threonine kinases such as MAPK and PKB (Bain et al.).

MAINTENANCE AND SELF-RENEWAL
· Maintains undifferentiated mouse ES cells in combination with PD0325901, in the absence of LIF (Ying et al.).
∙ Promotes self-renewal of human ES cells and mouse epiblast stem cells in combination with IWR-1 (Kim et al.).
∙ Allows derivation of ES cells from refractory mouse strains (Kiyonari et al., Ying et al.) and rat (Li P et al.) in combination with other small molecules.
· Maintains human and mouse hematopoietic stem cells in cytokine-free conditions, in combination with rapamycin (Huang et al.).
· Promotes growth of mouse and human intestinal stem cells (Wang et al.).

REPROGRAMMING
· Enables chemical reprogramming (without genetic factors) of mouse embryonic fibroblasts to iPS cells, in combination with Forskolin, Tranylcypromine, Valproic Acid, 3-Deazaneplanocin A, and E-616452 (Hou et al.).
∙ Promotes reprogramming of human somatic cells to iPS cells using OCT4, in combination with other small molecules (Zhu et al.).
∙ With OCT4, transdifferentiates human CD34+ hematopoietic cells to mesenchymal stem cells (Meng et al.).
· Direct lineage reprogramming of fibroblasts to mature neurons, in combination with Valproic Acid, RepSox, Forskolin, SP600125, Gö6983 and Y-27632 (Hu et al.).
· Direct lineage reprogramming of fibroblasts to mature neurons, in combination with Forskolin, ISX-9, SB431542, and I-BET151 (Li et al.).
· Generates mouse-like or “ground state” iPS cells from human and rat somatic cells, in combination with PD0325901 and A 83-01 (Li W et al. 2009).

DIFFERENTIATION
· Promotes differentiation of insulin-producing cells from human iPS cells (Kunisada et al.).
∙ Promotes differentiation of cardiomyocytes from human ES and iPS cells (Lian et al.).
∙ Generates and maintains primitive neural stem cells from human ES cells, in combination with SB431542 and Human Recombinant LIF (Li W et al. 2011).