Mono-ADP-ribosylation involves the transfer of the ADP-ribose moiety from NAD+ to a specific amino acid in the target protein. The rodent mono-ADP-ribosyltransferase RT6 is a glycosylphosphatidylinositol (GPI)-anchored membrane protein specifically expressed at the cell surface of rat and mouse T lymphocytes. The predicted 367-amino acid human ART3 protein has an estimated molecular mass of 41.5 kD and contains a hydrophobic peptide signal at its N terminus, 3 consensus motifs specific to enzymes catalyzing ADP-ribose transfer, a hydrophobic C-terminal sequence characteristic of a GPI-anchored protein, a novel motif repeated 3 times at its C terminus, and 1 potential glycosylation site.1 The ART3 and rodent RT6 proteins share 35% amino acid identity.