Overview of the cobblestone area forming cell (CAFC) assay and its relationship with other stem/progenitor cell assays. The CAFC assay permits the dissection of the hierarchical, heterogeneous stem cell pool into several distinct subsets. Most essential is the fact that with increasing duration of the culture period, the primitiveness of the evaluated cell subsets increases correspondingly. Shown are the CAFC subsets, evaluated at weekly intervals, and their equivalents in other classic, functional or phenotypic stem cell assays. Evaluating the cultures for 35 days covers the major part of the hematopoietic stem cell hierarchy. The question mark on top of the figure indicates that it is still possible that even more primitive cells exist. If this is the case, it has no implications for the conclusions drawn in this study. Abbreviations: S, a stem cell that has multi-lineage potential; P, a progenitor cell that has restricted potential; LTRA, a cell which has long-term repopulating abilities; CFU-S, colony forming unit-spleen.