The HI100 antibody reacts with an extracellular epitope on the CD45RA isoform of CD45, a type I transmembrane glycoprotein and member of the protein tyrosine phosphatase family (receptor class 1/6 subfamily). Alternative splicing of exons 4, 5, and 6 that encode the extracellular RA, RB, and RC polypeptides of CD45 gives rise to several isoforms with molecular masses of 180 – 240 kDa. Excision of exon 4 generates the ~220-kDa CD45RA isoform, which is expressed on naïve T cells, subsets of B cells and monocytes, and medullary thymocytes. CD45RA functions to enhance signaling through both the T cell receptor and B cell receptor. The HI100 antibody is commonly used in combination with antibodies against the CD45RO isoform to discern naïve and memory T cells and is useful for identifying the suppressor/inducer subset of CD4+ lymphocytes. The proportion of CD45RA+ T cells typically decreases with age.