1. Division of Reproductive Endocrinology & Infertility, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA
2. Assisted Conception Unit, 7th Floor, North Wing, St Thomas' Hospital, Lambeth Palace Road, London SE1 7EH, UK
3. Department of Cell and Molecular Pathology, St John's Institute of Dermatology, St Thomas' Hospital, Lambeth Palace Road, London SE1 7EH, UK
4. School of Biology, University of Leeds, Leeds LS2 9JT, UK