Project Summary Signals that cells receive over time from a small set of pathways (e.g., BMP, Wnt, and TGFβ) shape their fate and phenotype during development, regeneration, and disease. Despite their central importance, signaling histories of individual cells are often inaccessible to direct observation, hindering quantitative analysis and obscuring their connection to eventual cell fate. This challenge is particularly pronounced in mammalian systems, where limited optical access and the constraints of size and timescale often render live imaging impractical. To address this issue, we have developed an approach to reconstruct the history of signaling activity in single cells based on endpoint fluorescence images. This is achieved by regulating CRISPR base editors to generate mutations in engineered target sites at rates proportional to the signal of interest. These mutations create a heritable record of signaling activity in the genome, which can be read out at a later time, together with the gene expression profile of the cells. Using this approach, we demonstrated that cells retain a memory of their past response level to BMP signaling for up to 18 days, providing a mechanism for long-term interactions between signals that can facilitate coordination of developmental processes over time. In this proposal, we will expand the scope and utility of our signal recording approach by extending its dynamic range to capture the broad spectrum of in vivo signal intensities and enabling simultaneous recording of the sequence and timing of two signaling pathways. We will also engineer mouse embryonic stem cells to record three key developmental pathways: BMP, Wnt, and Nodal. This will allow us to generate stem cell-derived embryo models and chimeric embryos to link cell fate and spatial organization at the onset of organogenesis with signaling activity at different time windows earlier in development. Additionally, we will investigate mechanisms that enable long-term changes