1 Introduction Janus kinases (JAKs), including JAK1, JAK2, JAK3, and tyrosine kinase 2 (TYK2), are a family of intracellular tyrosine kinases that are involved in the signaling process of many cytokine receptors. The inhibition of JAKs is considered an important modulator of the JAK–STAT (signal transduction and activator of transcription) signaling pathway in immune-mediated diseases including rheumatoid arthritis, psoriasis, systemic lupus erythematosus, and inflammatory bowel disease [1, 2].