JAK1 protein, also known as Janus kinase 1, is a non-receptor tyrosine kinase that plays a crucial role in the JAK-STAT signaling pathway, which is involved in the transmission of signals from the cell surface to the nucleus in response to cytokines and growth factors. JAK1 is essential for the activation of STAT proteins, which are key regulators of gene expression. It is involved in the signaling of interferon-alpha/beta, interferon-gamma, and various cytokines, making it a central player in immune responses, inflammation, and hematopoiesis. JAK1 is also implicated in the development of several diseases, including autoinflammatory syndromes, immune dysregulation, and eosinophilia. Its dysregulation can lead to conditions such as breast implant-associated anaplastic large cell lymphoma. JAK1 is a therapeutic target for alleviating cytokine storms and is involved in the regulation of genes that mediate inflammation, epithelial remodeling, and metastatic cancer progression. The kinase activity of JAK1 is directly inhibited by the suppressor of cytokine signaling 1 (SOCS1) protein. Alternative splicing results in multiple transcript variants.