Homeobox genes encode transcription factors that regulate diverse developmental events. The largest known homeobox gene cluster – the X-linked mouse reproductive homeobox (Rhox) cluster – harbors genes whose expression patterns and functions are largely unknown. Here, we report that a member of this cluster,Rhox10, is expressed in male germ cells.Rhox10is highly transcribed in spermatogoniain vivoand is upregulated in response to the differentiation-inducing agent retinoic acidin vitro. Using a specific RHOX10 antiserum that we generated, we found that RHOX10 protein is selectively expressed in fetal gonocytes, germline stem cells, spermatogonia, and early spermatocytes. RHOX10 protein undergoes a dramatic shift in subcellular localization as germ cells progress from mitotically arrested gonocytes to mitotic spermatogonia and from mitotic spermatogonia to early meiotic spermatocytes, consistent with RHOX10 performing different functions in these stages.