CD25 is expressed broadly among leukocytes. The highest surface expression of this protein is on regulatory T cells (Tregs), where CD25 is expressed constitutively, especially on a subset classified as naturally occurring Tregs. It can also be found on activated B cells, NK (natural killer) cells, thymocytes, and some myeloid lineage cells (e.g. macrophages, dendritic cells). L2RA has been used as a marker to identify CD4+FoxP3+ regulatory T cells in mice, and it has been found that a large proportion of resting memory T cells constitutively express IL2RA in humans. Levels of CD25 soluble form, called sIL-2Rα, has been connected to pathogenesis of autoimmune diseases and cancer. Since sCD25 is produced during immune activation, it is used as one of biomarkers to track disease progression and to indicate outcome for clinical disorders. Especially, it is a hallmark for hyper-activated immune system and cytokine storm, which may lead to multiple organ system failure.