The mammalian brain contains many specialized cells that develop from a thin sheet of neuroepithelial progenitor cells. However, the lineage relationships between mature cell types and progenitor cells are not well understood. Therefore, researchers performed high-throughput clonal tracking and expression profiling of mouse forebrain cells using single-cell and spatial transcriptomics and found two populations of fate-restricted progenitor cells present as early as embryonic day (E) 9.5 in the murine hippocampus. They found that microglia are generated from a limited number of progenitor cells that undergo massive clonal expansion as well as widespread migration throughout the mouse telencephalon. The results thus demonstrate the utility of high-throughput clonal tracing in the mouse brain to provide molecular information about brain development at the single cell and tissue level. TREX is the technique that allowed them to profile TRacking and gene EXpression of clonally bound cells in the mouse brain by high-throughput single-cell RNA sequencing (scRNA-seq). TREX is based on a diverse library of lentiviruses containing barcodes downstream of a GFP protein.