Dr. Klein is fascinated by the question of how stem cells choose between alternative fates in developing and adult tissues. Today, one can strip differentiated cells of their fate by inducing pluripotency, yet it remains exceedingly difficult to target differentiating stem cells to a specific fate. His work focuses on the stem cells that maintain adult tissues such as the skin, and more recently on embryonic stem cells. Integral to his research is the use of theoretical tools to guide the conception, design and analysis of experiments. Much of the theory used in his lab is inspired by stochastic physics and mathematical population dynamics, which provide the necessary language to describe the diversity of fate outcomes that emerge within a cell population. Dr. Klein has recently developed a powerful new experimental method to obtain a detailed profile of the state of thousands of individual cells, which he is using to dissect the causes and functional consequences of heterogeneity in stem cell populations.