A new way to build cell lineages

The identity or state of a cell depends on numerous factors. Some of these factors are transient in nature (such as the stage the cell is at in the cell cycle), while others reflect long-lasting commitments, such as those that occur during the development of stem cells (Novershtern et al., 2011, Graf and Enver, 2009). By making the entire transcriptome available, single-cell RNA sequencing is now allowing researchers to systematically investigate these factors (Wagner et al., 2016; Tanay and Regev, 2017). Specifically, single-cell technology opens the way for developmental biologists who work on the transitions between different cell states to explore three outstanding questions: (1) What are the cell states (both transitional and long lasting or terminal) that comprise a developmental process of interest? (2) What transitions take place between these states? (3) How are these transitions regulated?