Interleukin-2 (IL-2), encoded by the mouse Il2 gene, is a secreted T-cell growth factor orthologous to human IL-2. It is produced predominantly by activated CD4+ helper T cells and, to a lesser degree, by activated CD8+ T cells and NK cells, and is secreted into the extracellular space. Mouse IL-2 binds the interleukin-2 receptor, assembled from the IL-2Ralpha chain (CD25), IL-2Rbeta chain (CD122), and the common gamma chain (CD132); the trimer forms the high-affinity receptor, while CD122/CD132 dimers form the intermediate-affinity receptor. Ligand binding activates receptor-associated JAK1 and JAK3 kinases, which phosphorylate STAT5a/STAT5b; STAT5 dimers translocate to the nucleus and upregulate genes driving T-cell expansion, cytotoxic lymphocyte activity, B-cell help, and Foxp3-dependent regulatory T-cell development and homeostasis, alongside PI3K/AKT and MAPK branches. IL-2 is essential for thymic and peripheral Treg maintenance and for immune tolerance. It is a soluble secreted cytokine.