Mono(ADP-ribosyl)transferase (EC 2.4.2.31) catalyzes the transfer of the ADP-ribose moiety of nicotinamide adenine dinucleotide (NAD) to an acceptor amino acid in proteins. In vertebrates there is a family of such enzymes that transfer an NAD group to arginine [1].

At least five distinct forms of this enzyme has been found so far, some are attached to the membrane by a GPI anchor while others seem to be secreted. They are proteins of about 250 to 300 amino acid residues. As a signature pattern, we selected a conserved region in the central section of these enzymes.