IL-2（Cys145Ser） is produced by recombinant DNA technology using a genetically engineered E. coli strain containing an analog of the human interleukin-2 gene. It has been engineered to contain the serine for cysteine substitution at amino acid position 145. IL-2（Cys145Ser） has similar characteristics and functions to IL-2, which is a leukocytotrophic hormone that is instrumental in the body natural response to microbial infection and in discriminating between foreign （non-self） and self. IL-2 is expressed by CD4+ and CD8+ T cells, gamma δ T cells, B cells, dendritic cells, and eosinophils. Mature human IL-2 shares 56% and 66% amino acid （aa） sequence identity with mouse and rat IL-2, respectively. Human and mouse IL-2 exhibit cross-species activity. IL-2 can stimulate B-cells, monocytes, lymphokine-activated killer cells, natural killer cells, and glioma cells, and plays a vital role in the expansion and maintenance of regulatory T cells, although it inhibits the development of Th17 polarized cells. Thus, IL-2 may be a key cytokine in the natural suppression of autoimmunity. Recombinant IL-2 is widely used in cell culture for the expansion of T cells.