Objective: PATZ1 is a recently discovered zinc finger protein that due to the presence of the POZ domain acts as a tranh1ional repressor affecting the basal activity of different promoters. To gain insights into its biological role we generated mice lacking the PATZ1 gene. Male PATZ1-/- mice were unfertile suggesting a crucial role of this gene in spermatogenesis. Consistently most of adult testes from these mice showed only few spermatocytes associated with increased apoptosis and complete absence of spermatids and spermatozoa with the subsequent loss of tubular structure. Materials and Methods: The analysis of PATZ1 expression by Northern blot Western blot and immunohistochemistry revealed its presence in Sertoli cells and among the germ cells exclusively in the spermatogonia. Results: Since PATZ1 has been indicated as a potential tumour suppressor gene we also looked at its expression in tumours deriving from testicular germ cells (TGCTs). Although expression of PATZ1 protein was increased in these tumours it was delocalized in the cytoplasm suggesting an impaired function. Conclusion: These results indicate that PATZ1 plays a crucial role in normal male gametogenesis and that its up-regulation and mis-localization could be associated to the development of TGCTs.