In order to clarify general mechanisms of T cell development, we used array technology to identify genes differentially expressed between wild type and mutant mice thymuses. This quantitative differential screening leads to the isolation of Spatial, a putative transcription factor, expressed at high level in thymic cortical stromal cells. We report here, by RT-PCR on 48 distinct tissues that Spatial is also highly expressed in testis. Interestingly, the testicular expression is developmentally regulated, since it only appears in adult mice around 7-8 weeks of age. Analysis of purified testicular cell types show that Spatial expression is restricted to haploid round spermatids during spermiogenesis and the expression sites were further localized by in situ hybridization to step 2-10 spermatids. Thus, this recently described nuclear factor constitutes a new marker of the round spermatid stage that may play a key role in the control of male germ cell development.