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Definition of JAK3
Medically reviewed by Min Clinic Staff | Updated: January 2026
JAK3: Or Jak3. An enzyme found only in cells in the immune system that is critical for the cell signaling process resulting in the development of white blood cells. Mutation of the gene encoding JAK3 is responsible for a form of severe combined immunodeficiency (SCID). JAK stands for Janus kinase or Just Another Kinase.