Tyrosine kinase 2 (TYK2), expressed in astrocytes and microglia, is involved in the activation of pathways triggered by proinflammatory cytokines, such as IL-23, IL-12 and type I interferons (IFNs), within the central nervous system (CNS). Dysregulated activation of astrocytes and microglia may contribute to the neuroinflammation associated with progressive forms of multiple sclerosis (MS).