MRC Mammalian Genetics Unit, Harwell, Oxfordshire OX11 0RD, UK. e.green@har.mrc.ac.uk|MRC Mammalian Genetics Unit, Harwell, Oxfordshire OX11 0RD, UK. e.green@har.mrc.ac.uk|MRC Mammalian Genetics Unit, Harwell, Oxfordshire OX11 0RD, UK. e.green@har.mrc.ac.uk|MRC Mammalian Genetics Unit, Harwell, Oxfordshire OX11 0RD, UK. e.green@har.mrc.ac.uk|MRC Mammalian Genetics Unit, Harwell, Oxfordshire OX11 0RD, UK. e.green@har.mrc.ac.uk|MRC Mammalian Genetics Unit, Harwell, Oxfordshire OX11 0RD, UK. e.green@har.mrc.ac.uk