Acknowledgements
We thank L. de Wit for help during tissue collections, G.J.F. Overkamp for technical assistance, Prof.dr. M.P. Gerkema for establishing the common vole colony and Dr. C. Dijkstra and Dr. L. van de Zande for establishing the tundra vole colony at the University of Groningen. This work was funded by the Adaptive Life program of the University of Groningen (B050216)