The tracing of HSC barcodes introduced in situ supports a treelike model of hematopoiesis with major myeloerythroid and common lymphoid branches. (A) We introduced barcodes during fetal development in the mouse (embryonic day 9.5). Labeled clones of adult HSC (analyzed at ∼1 year of age) are of very different sizes. HSC clones typically yield multilineage progeny, or oligolineage progeny either on the myeloerythroid or on the lymphoid side. (B) When barcoding HSCs in adult mice, HSCs again give rise to multilineage or oligolineage (typically, myeloerythroid or lymphoid) progeny. In addition, HSC-derived barcodes are also found in more restricted sets of mature cells (bi- or unilineage). Overall, HSC-derived barcodes co-occur in mature lineages and in their respective progenitors. For example, barcodes enriched in erythroid progenitors (EryP), granulocytes (Gr), or monocytes (Mono) are likely to be enriched also in CMPs/GMPs, whereas barcodes prevalent in mature T and B cells are also prevalent in the respective progenitors but not in CMPs. Colors symbolize the genetic Polylox barcodes on which this figure is based.¹¹  EryP, erythrocyte progenitor; preB, precursor B cell; preT, precursor T cell.