Major advances in pharmacology of the 21st century include the development of a new class of drugs, which are low-molecular, chemically synthesized molecules (the so-called "small molecules"), the point of application of which is Janus kinase (Janus kinase, JAK) involved in intracellular cytokine signaling. The review examines the molecular aspects of the JAK-STAT signaling pathway, justifying the use of the JAK-kinase inhibitor (tofacitinib) in the treatment of inflammatory bowel disease.