In 1995, Stephen was appointed an Assistant Professor in Neurology at The University of Kentucky Medical School. He moved his stem cell research programme to Guy’s Hospital, London in 1996 and was appointed a Lecturer in Biomolecular Sciences at King’s College London in 1998. Over the last 13 years, his research group has worked with a wide range of somatic stem cell populations, as well as mouse and human embryonic stem (ES) cells. In 2002, together with Dr. Susan Pickering and Professor Peter Braude, Stephen was awarded one of the first two licenses granted by the Human Fertilisation and Embryology Authority for the derivation of human ES cells. His group subsequently generated the first human embryonic stem cell line in the UK and was one of the first groups to deposit this into the UK Stem Cell Bank. They have gone on to generate three new human ES cell lines, including one that encodes the most common genetic mutation resulting in Cystic Fibrosis.