Human cells express at least eight members of the MutT motif protein (or nudix hydrolase) family. These enzymes are believed to eliminate toxic nucleotide derivatives from the cell and regulate the levels of important signalling nucleotides and their metabolites. Six have been fully or partially characterized: i) hMTH1 is a nucleoside triphosphatase which restricts AT-->CG transversions by specifically degrading the oxidized nucleotide 8-oxo-dGTP; ii) hAPAH1 preferentially degrades the signalling dinucleotide Ap4A; iii) DIPP is unusual in hydrolysing two seemingly unrelated signalling substrate groups - the dinucleotides Ap6A and Ap5A, and the diphosphoinositol polyphosphates; iv) DIPP2 is closely related to DIPP; v) hYSAH1 is an NDP-sugar hydrolase which prefers ADP-ribose, and vi) hGFG is a protein of unknown function encoded by the antisense transcript of the basic fibroblast growth factor gene. Although not yet associated with known hereditary or acquired disorders, the functional loss of any one of these hydrolases would be expected to be detrimental to cellular function. Furthermore, the ialA invasion gene of Bartonella bacilliformis and other invasive pathogens encodes a MutT motif Ap4A hydrolase while poxviruses express two MutT motif proteins, at least one of which is essential for infectivity. This protein family, therefore, occupies a position of some importance in controlling human health and disease.