The A20 antibody reacts with mouse CD45.1, also known as Ly5.1, which is a strain-specific allelic form of the CD45 Leukocyte Common Antigen (LCA). Functionally, CD45 is a protein tyrosine phosphatase whose broad cell distribution supports a critical role in many leukocyte functions, including regulation of signal transduction and cell activation associated with the T cell and B cell receptors.

The A20 antibody is typically used as a leukocyte marker in Ly5.1 mouse strains: SJL/J, DA, STS/A and RIII. The antibody has been demonstrated to specific for CD45.1 and is not cross-reactive with CD45.2 bearing cells.