Project Summary/Abstract The epigenetic machinery is essential for mammalian development, and components of this machinery are mutated in a variety of human diseases. Hundreds of regulatory proteins participate in the reading, writing, and erasing of epigenetic information over the lifespan of an organism. However, the in vivo activity of many of these epigenetic regulators remain inadequately characterized. Here, we leverage IMPC-generated mouse lines lacking epigenetic regulators, together with recent advances in single-cell epigenomic and transcriptomic technologies, to interrogate components of the epigenetic machinery in vivo at the molecular level, and with the cell-type resolution necessary to understand their complex roles in development and disease. The data produced here will reveal gene regulatory defects caused by loss of epigenetic regulators in a diverse panel of tissues and cell types relevant to human congenital malformations. This tissue- and cell-type-resolved view of the effects of mutations in the epigenetic machinery is a critical step in larger efforts to reveal mechanisms of epigenetically- influenced disease risk, interpret non-coding sequence variation, and ultimately to tailor diagnostics and therapeutics to the epigenomic and transcriptomic states of a patient’s disease-contributing cells.