Peter Mazur was a researcher in the US who developed new ways of preserving biological material by freezing it, a process called cryopreservation. If done correctly, cryopreservation enables scientists to store or study biological material for an extended period of time. If done incorrectly, cryopreservation can easily harm or destroy biological material. Mazur worked to find the best ways to cryopreserve different cells, embryos, and organs in order to minimize the damage caused by freezing. Throughout the 1960s and 1970s, Mazur and his colleagues published a series of papers that ultimately led to the discovery of previously unexplored factors that can cause harm to cells during the cryopreservation process. He called that discovery the two-factor hypothesis. That same year, Mazur also contributed to one of the first successful attempts at cryopreserving viable mouse embryos. Mazur’s work to improve the cryopreservation process helped to establish foundational knowledge that was later used in many different fields, such as reproductive health and conservation.