In many cancers, such as ovarian cancer, each round of chemotherapy kills the majority of cancer cells, while a small population of them survives through treatment. These cells are typically more resistant for the next cycle of therapy and can thus regrow to a deadly, treatment resistant tumor. Researchers wanted to know how this small population of surviving cells differs from the other more sensitive cells already before the treatment. To enable this cellular time travel, they developed ReSisTrace, a methodology that takes advantage of the similarity of sister cells to trace back pre-existing treatment resistance in cancer.