Abstract from Parent Grant DNA recording is a recently developed technology that allows cellular signaling events of interest to be followed over time without removing cells from their physiological context. During a DNA recording experiment, genetically engineered cells expressing the DNA recorder acquire mutations at a “recording locus” as the result of transient cellular events, without perturbing their normal phenotype. At the end of the experiment, once their eventual phenotype is known, cells can be collected and their histories determined by sequencing the recording locus. The work proposed here will lead to a DNA recorder that is not limited, as current technology is, to recording one or two cellular events per experiment, but can record dozens of events in parallel, allowing an unbiased examination of cell history in any context compatible with genetically engineered cells. In this project, the new DNA recorder will be used to study long-term effects for a cell of experiencing significant spontaneous DNA damage. Although DNA damage is very well studied, it has not previously been possible to follow the rare cells that experience significant spontaneous damage, especially in an in vivo mammalian context. The new DNA recorder will be developed in stages: First, intracellular recording machinery, will be created for a small number of cellular events associated with the DNA damage response. Next, the fidelity of this new recording machinery will be extensively validated in cultured cells, including head and neck cancer cells becoming resistant to the DNA-damaging chemotherapeutic cisplatin. Finally, the recorder will be validated in mouse embryonic stem cells, and used to test how spontaneous DNA damage affects cell fate determination during the earliest stages of development.