The focus of his work is the application of knowledge emerging from the Human Genome Project to the identification of risk factors, disease processes and new treatments for common disorders.

In psychiatric genetics, his group has identified several genes of major effect in determining the risk of developing schizophrenia or bipolar affective disorder, most notably DISC1, now recognized as one of the best validated and informative findings in the field (Millar et al, Science, 2005, Science Magazine Scientific Breakthrough of the Year).

In cystic fibrosis gene therapy, his contribution started with development of a transgenic mouse model of the disease (Dorin et al, Nature, 1992 and MRC Principal Achievement, 1992), continued with rescue of the biochemical defect in the CF mutant mice by gene therapy, leading directly to the first UK clinical trial of non-viral gene therapy for cystic fibrosis (Nature Medicine, 1995) and the first clinical trial in Scotland. In 2001, his group joined with Imperial College London and Oxford University to form the UK Cystic Fibrosis Gene Therapy Consortium to develop and apply the next generation of clinical gene therapy.