He now leads a research unit involved in international and national studies on the treatment of HIV and hepatitis C and was the Secretary of the BHIVA Hepatitis Working Group Executive Panel. He is a member of the BHIVA Executive Committee) and Scottish HIV Advisory Group (SHIVAG) executive committees and has been on the BHIVA Treatment Guidelines Writing Committee since 2003. He was appointed on the Expert Advisory Group on AIDS, and was a member of the UK Medical Research Council College of Experts Consortium. He is Training Programme Director for Infectious Diseases in Scotland. Professor Leen chaired the BHIVA Education and Scientific Subcommittee from 2008 to 2011. He is a member of the management group of Edinburgh University’s Centre for Infectious Diseases since 2008.