Forming the early heart The heart is the first organ to form during development and is critical for the survival of the embryo. The precise molecular identities of the various cell types that make up the heart during these early stages remain poorly defined. Tyser et al. used a combination of transcriptomic, imaging, and genetic lineage–labeling approaches to profile the molecular identity and precise locations of cells involved in the formation of the mouse embryonic heart. This approach allowed them to identify the earliest known progenitor of the epicardium, the outermost layer of the heart, which is an important source of signals and cells during cardiac development and injury. Science , this issue p. eabb2986