With the exception of brain, most human tissues analysed contain dUTPase protein detectable by immunohistochemistry. Non-dividing tissues like untreated peripheral blood lymphocytes (PBLs) contain basal levels of cytoplasmic dUTPase and express additional, nuclear dUTPase upon mitogenic stimulation. Normal, proliferating tissues like intestinal mucosa or germinal centres within tonsils contain cytoplasmic as well as nuclear dUTPase in accordance with a proposed role for dUTPase during cell division. Notably, no dUTPase is detectable during mitosis. The failure to stain dUTPase in normal brain tissue by immunohistochemistry while mRNA is readily detectable by Northern blotting cannot be explained at this moment. Epithelial tumours, such as adenocarcinoma of the lung, breast, colon, vulva or nasopharynx contain cells which are either positive for both or only one of the subcellular forms of the dUTPase and show variable numbers of dUTPase-positive cells. High levels of dUTPase correlate with a poor p rognosis regarding the progression of colorectal carcinoma. Of the intracranial tumours tested, neuroepithelial tumours show almost exclusively nuclear expression whereas meningiomas of higher grades of malignancy (WHO grade II and III) also contain cells with additional cytoplasmic dUTPase. The dUTPase is detectable in other malignancies including tumours derived from lymphatic tissues like Burkitts lymphoma or non-Hodgkins-lymphoma. The downregulation of dUTPase protein during apoptosis or the inhibition of dUTPase during nerve cell development in Drosophila melanogaster suggests a possible role of the enzyme during apoptosis. In line with these observations, inhibition of dUTPase by antisense in p53-deficient tumour cells hints at a possible route of treatment of p53-deficient tumours which are otherwise resistant to therapies like irradiation. The expression of the enzyme in normal tissues indicates that sublethal levels of dUTPase inhibitors may also exert an unwanted mutagenic effect.