The mole vole Ellobius lutescens is an interesting animal, not only concerning its sex determination mechanism without the Y-chromosomal Sry gene, that triggers sex determination in nearly all other mammalian species, but also regarding the karyotype with an odd number of chromosomes, being identical in male and female animals. The odd chromosome represents the X chromosome, and therefore, even males do not have a Y chromosome. We present an overview of a search for candidate genes of male sex determination in the mole vole Ellobius lutescens. A singular X raises questions about the need for X chromosome inactivation in female cells. We present preliminary data that support a hypothesis that the E. lutescensXist gene may be degenerated and thus non-functional.