Lymphocyte-activation gene 3, also known as LAG-3, is a protein which in humans is encoded by the LAG3 gene. LAG3, which was discovered in 1990 and was designated CD223 (cluster of differentiation 223) after the Seventh Human Leucocyte Differentiation Antigen Workshop in 2000, is a cell surface molecule with diverse biologic effects on T cell function. It is an immune checkpoint receptor and as such is the target of various drug development programs by pharmaceutical companies seeking to develop new treatments for cancer and autoimmune disorders. In soluble form it is also being developed as a cancer drug in its own right. Gene The LAG3 gene contains 8 exons. The sequence data, exon/intron organization, and chromosomal localization all indicate a close relationship of LAG3 to CD4. The gene for LAG-3 lies adjacent to the gene for CD4 on human chromosome 12 (12p13) and is approximately 20% identical to the CD4 gene. Protein The LAG3 protein, which belongs to immunoglobulin (Ig) superfamily, comprises a 503-amino acid type I transmembrane protein with four extracellular Ig-like domains, designated D1 to D4. When human LAG-3 was cloned in 1990 it was found to have approx. 70% homology with murine LAG3. The homology of pig LAG3 is 78%. Tissue distribution LAG-3 is expressed on activated T cells, natural killer cells, B cells and plasmacytoid dendritic cells. Function LAG3's main ligand is MHC class II, to which it binds with higher affinity than CD4. The protein negatively regulates cellular proliferation, activation, and homeostasis of T cells, in a similar fashion to CTLA-4 and PD-1 and has been reported to play a role in Treg suppressive function. Fibrinogen-like protein1 FGL1, a liver-secreted protein, is another (major) LAG3 functional ligand independent of MHC-II. LAG3 also helps maintain CD8+ T cells in a tolerogenic state and, working with PD-1, helps maintain CD8 exhaustion during chronic viral infection. LAG3 is known to be involved in the maturation and activation of dendritic cells. Use as a pharmaceutical and as a drug target There are three approaches involving LAG3 that are in clinical development. The first is IMP321, a soluble LAG3 which activates dendritic cells. The second are antibodies to LAG3 which take the brakes off the anti-cancer immune response. An example is relatlimab, an anti-LAG3 monoclonal antibody that is currently in phase 2 clinical testing. A number of additional LAG3 antibodies are in preclinical development. LAG-3 may be a better checkpoint