Technologies such as RNA sequencing are now revealing which genes are expressed in each individual cell. All cells can then be arranged systematically using similar expression profiles. Dr. Jan Philipp Junker, head of the Quantitative Developmental Biology research group at the Max Delbrück Center for Molecular Medicine in the Helmholtz Association (MDC), says, “Whenever we use such a technology to examine an organ or an organism, we find not only familiar cell types, but also unknown and rare ones. The next question is obvious—where do these different types come from?” Junker’s team is has now published a paper in Nature Biotechnology that describes a technique called LINNAEUS that enables researchers to determine the cell type as well as the lineage of each cell.