X-inactivation in female mammals, whereby one of the two X-chromosomes is genetically switched off, is one of the unique and enigmatic phenomena in genetics. This process has been studied for more than thirty years and yet the molecular mechanisms regulating the process of X-inactivation remain unclear. The understanding of this unusual phenomenon will have implications in many related fields; these include understanding the mechanisms underlying the evolution of the mammalian X-chromosome; understanding the means whereby genes can be regulated by higher-order chromatin structure; and understanding the structure and organisation of chromosomes in complex organisms. The recent discovery of the XIST gene located in the inactivation centre region of the X-chromosome, and known to be expressed only on the inactive X-chromosome in man and mouse, has led to some progress towards understanding the steps involved in X-inactivation.

Location, expression and molecular structure of the XIST gene in 4 vole species of the genus Microtus will be studied. Previous results indicate that the vole is an excellent model for research on X-inactivation and may provide answers not readily available in other models such as man and mouse. Voles are unique because of the specific location of heterochromatin blocks on the X-chromosomes in four Microtus vole species, and the associated nonrandom X-inactivation in hybrids of the genus Microtus. It will be of great interest to study the XIST gene in this unique biological system and the results can be expected to shed further light on the function of XIST in X-inactivation.