Janus kinases (JAKs) are a family of four enzymes inside your cells that act as signal relays. When a chemical messenger like a cytokine docks on the outside of a cell, JAKs translate that signal into action inside the cell, telling it to grow, divide, fight infection, or produce blood cells. Because they sit at this critical relay point, JAKs play a role in immune function, blood cell production, and inflammation. When they malfunction, the consequences range from autoimmune diseases to blood cancers.
How JAKs Work: The JAK-STAT Pathway
JAKs don’t float freely inside cells. They’re tethered to the inner surface of cytokine receptors, proteins that span the cell membrane and act like antennas for outside signals. The signaling chain works in a specific sequence. First, an extracellular molecule (a cytokine or hormone) binds to its receptor on the cell surface. This binding causes the receptor to change shape, bringing two JAK enzymes close enough together to activate each other through a process called phosphorylation, where one JAK transfers a phosphate group to the other.
Once activated, the JAKs phosphorylate the receptor itself, creating docking sites for a second group of proteins called STATs (signal transducers and activators of transcription). STATs latch onto these sites, get phosphorylated by the JAKs, then pair up and travel into the cell’s nucleus. There, they bind to specific stretches of DNA and switch target genes on or off. This entire relay, from the cytokine hitting the receptor to genes being activated, is called the JAK-STAT pathway. It controls processes as varied as immune cell maturation, inflammation, and red blood cell production.
The Four JAK Enzymes
There are four members of the family: JAK1, JAK2, JAK3, and TYK2. All four share a similar structure, including a critical pocket where ATP (the cell’s energy molecule) binds. That ATP supplies the phosphate group JAKs need to do their job. While they share this architecture, each JAK partners with different cytokine receptors, which means each one influences different biological processes.
JAK1 and JAK2 are expressed broadly across many tissue types and participate in signaling for a wide range of cytokines involved in immunity and blood cell formation. JAK3 is more restricted, found mainly in immune cells, making it especially important for immune system development. TYK2 is involved in signaling for interferons and certain inflammatory cytokines. In practice, most cytokine receptors use a combination of two JAKs working together rather than a single one acting alone.
JAK2 Mutations and Blood Disorders
The most well-known medical consequence of a JAK malfunction is the JAK2 V617F mutation, identified in 2005. This single amino acid change leaves JAK2 permanently switched on, driving the overproduction of mature blood cells. It is the defining genetic feature of a group of blood cancers called myeloproliferative neoplasms (MPNs).
The mutation is found in roughly 95% of people with polycythemia vera, a condition where the body makes too many red blood cells, and in 50 to 60% of those with essential thrombocythemia (too many platelets) or myelofibrosis (scarring in the bone marrow). A smaller subset of polycythemia vera patients, about 3 to 5%, carry different mutations in a nearby region of the JAK2 gene called exon 12.
Because the mutation is so strongly linked to these conditions, the World Health Organization includes JAK2 mutation testing as a major diagnostic criterion. For polycythemia vera, the presence of a JAK2 mutation is one of three major criteria required for diagnosis. For essential thrombocythemia and myelofibrosis, testing for JAK2 alongside two other driver mutations (CALR and MPL) is a standard part of the diagnostic workup. A higher burden of the JAK2 V617F mutation in blood samples can also help doctors distinguish between subtypes when the clinical picture is unclear.
JAK Inhibitors as Treatment
Because JAKs depend on ATP binding to function, drugs called JAK inhibitors (sometimes called “jakinibs”) work by physically blocking the ATP binding pocket on the enzyme. Without ATP, the JAK can’t transfer phosphate groups, and the entire signaling cascade stalls. There are over 500 similar enzymes in the human genome that use related ATP pockets, which is one reason designing a JAK inhibitor that hits only the intended target has been a challenge.
Several JAK inhibitors have received FDA approval for a range of conditions:
- Ruxolitinib targets JAK1 and JAK2. It is approved for polycythemia vera and myelofibrosis, the blood cancers driven by overactive JAK2 signaling.
- Tofacitinib was the first JAK inhibitor approved for an autoimmune condition, receiving FDA clearance for rheumatoid arthritis in 2012. It has since been approved for psoriatic arthritis, ulcerative colitis, ankylosing spondylitis, and juvenile idiopathic arthritis.
- Baricitinib was approved for rheumatoid arthritis in 2018 and later authorized for hospitalized COVID-19 patients.
- Upadacitinib was approved in 2019 for moderate to severe rheumatoid arthritis and later expanded to psoriatic arthritis.
Risks of JAK Inhibitors
Because the JAK-STAT pathway controls so many immune and blood cell functions, blocking it comes with real tradeoffs. In 2021, the FDA added its strongest safety label, a boxed warning, to several JAK inhibitors (tofacitinib, baricitinib, and upadacitinib). The warning was based on a large randomized trial of tofacitinib that found increased risks of serious heart-related events like heart attack and stroke, blood clots, certain cancers including lymphoma and lung cancer, and death compared to another class of arthritis drugs.
These risks don’t mean JAK inhibitors are unsafe for everyone, but they do mean doctors typically consider them after other treatments have been tried. The risk profile also varies depending on the specific drug and which JAKs it targets, which has pushed drug development toward more selective inhibitors.
Newer, More Selective Inhibitors
Earlier JAK inhibitors block multiple JAK family members at once, which controls disease effectively but also explains the broader side effects. Newer drugs aim to hit only one JAK, reducing off-target effects. Ritlecitinib, for example, is a covalent inhibitor with 900- to 2,500-fold selectivity for JAK3 over the other three family members, specifically suppressing one branch of immune signaling while leaving others largely intact.
Deucravacitinib takes an entirely different approach. Instead of blocking the ATP pocket that all four JAKs share, it binds to a regulatory region called the pseudokinase domain, which is unique to TYK2. In lab testing, it did not inhibit any of the other JAK enzymes, giving it a narrow signaling footprint. It selectively blocks interferon-alpha signaling and is approved for psoriasis. This shift from broad to targeted inhibition represents the direction JAK inhibitor development is heading, aiming to keep the therapeutic benefits while minimizing the cardiovascular and cancer risks that come with shutting down the pathway too broadly.