Stem cells are vital to human health. But we still don’t know the answers to some of the most basic questions about them—such as how many populate our bodies and how they change with age. Leif, a physician-scientist in the Regev Lab, intends to answer these questions by tracking lineages of mutations in human stem cells, similar to building a family tree, using genome sequencing. His work could open new doors for understanding how stem cells contribute to cancer and other blood-related diseases.