Centre for Asthma and Respiratory Diseases, School of Biomedical Sciences, University of Newcastle, Newcastle; Monash Institute of Medical Research, Monash University, Clayton; Gene Targeting Group, John Curtin School of Medical Research, Australian National University, Canberra, Australia; and Stem Cell Unit, Department of Anatomy, College of Medicine, and King Khalid University Hospital, King Saud University, Riyadh, Saudi Arabia