Polycystic kidney disease, or PKD, is the most common life-threatening genetic disease. It is caused by mutations in one of two genes. Though the genetic defect that causes PKD is known, how it leads kidneys to enlarge and choke with fluid-filled cysts has been frustratingly elusive. A new line of evidence points to a cellular appendage called the primary cilium, which may act as a mechanical sensor in cells lining kidney ducts. The latest study, led by Jing Zhou, Harvard Medical School associate professor of medicine at Brigham and Women’s Hospital, and published online in the Jan. 6, 2003 Nature Genetics, shows that the two genes involved in PKD help sense fluid flow across the primary cilium of kidney cells. Although still preliminary, the work is bringing a new focus in PKD research and turning attention to the role of mechanical sensation in cells.