The cDNA and derived amino acid sequence of human diadenosine 5', 5'''-P-1, P-4-tetraphosphate pyrophosphohydrolase have been determined with the aid of the GenBank Expressed Sequence Tag database. This enzyme possesses a modification of the MuT sequence motif found in certain nucleotide pyrophosphatases. It is unrelated to the enzymes of diadenosine tetraphosphate catabolism found in prokaryotes and fungi.