We searched the database of expressed sequence tags (dbEST) for relatives of the known human and murine mono(ADP-ribosyl)transferases (mADPRT), poly(ADP-ribosyl)polymerases (PARP), ADP-ribosyl cyclases, and ADP-ribosylarginine hydrolases (ARH). By May 31, 1996, all of the known enzymes except for RT6 were represented in dbEST by exact sequence matches from mouse and/or human tissues. Several ESTs show significant sequence similarity but not identity to known mADPRTs. We isolated, cloned, and sequenced the corresponding genes. Our results show that seven human ESTs stem from a novel gene, provisionally designated LART, which is specifically expressed in lymphatic tissues. Five human ESTs stem from a novel gene, here designated TART1, which is specifically expressed in testis. This gene is also represented by a single mouse EST. One other mouse EST stems from a distinct gene, here designated TART2, which is also expressed in testis. These genes have similar exon/intron structures. The predicted LART and TART1 gene products contain hydrophobic N- and C-terminal signal peptides characteristic for GPI-anchored surface proteins, TART2 lacks the GPI-anchor signal peptide. The predicted native proteins show 28-42% sequence identity to one another. They each contain four cysteine residues that probably form conserved disulfide bonds. They each also contain a conserved glutamic acid residue within the proposed active site motif LART and TART1 show interesting deviations from the surrounding consensus sequence.