Immortalisation of bovine leukocytes by the macroschizont stage of the tick transmitted protozoan parasite, Theileria annulata, results in the clonal expansion of infected cells and dissemination throughout the bovine host. The parasite-encoded factors which induce this unique transformation event have not been defined to date. In this study, a gene family (TashAT) has been characterised that encodes polypeptides with homology to known DNA-binding proteins. Expression of TashAT genes occurs at the intracellular macroschizont stage of the parasite life cycle and during differentiation to the merozoite, negative regulation of TashAT genes is detected early relative to other macroschizont genes. Interestingly, the early reduction in TashAT expression coincides with the initial decrease in host cell proliferation. One member of the family, TashAT2, was characterised in detail and the predicted polypeptide sequence was found to harbor three AT hook DNA-binding domains. Antisera generated against two distinct regions of TashAT2 both located the antigen to the host cell nucleus and, combined with protein translation inhibition and immunoprecipitation studies, provide evidence that this polypeptide could be transported from the parasite to this location. Further evidence for this postulation was provided by transfection studies which demonstrated that TashAT2 does encode the structural information required for localisation to the nucleus of a mammalian cell. Thus, TashAT2 is a potential candidate for a parasite-encoded factor that modulates host cell gene expression and may be involved in the control of host cell proliferation.