In the past year we have used CIRM funding to discover how the physical properties of a stem cell’s surroundings affect its growth, proliferation, and ability to turn into other cell types. Previous research shows that stem cells grown on soft surfaces grow into fat cells, while stem cells grown on hard surfaces grow into muscle or bone cells. At present we do not know how stem cells sense the stiffness of their surroundings. We created a protein that changes color when it is stretched, and used this protein to measure the forces inside living stem cells for the first time. We are using this protein to discover how stem cells sense mechanical forces, both from the surrounding tissue and from neighboring cells. Understanding this scientific question will help scientists to grow large numbers of stem cells, a critical bottleneck in regenerative medicine. In addition, it will help tissue engineers understand how to construct tissues and organs to replace those damaged by disease or injury. We are grateful to the citizens of California for their support and look forward to exciting discoveries in the coming year.