Observed vs potential differentiation of multipotent progenitors. Cells behave differently, depending on the experimental system employed. The absence of a particular lineage read-out in any specific assay cannot correctly be interpreted as a lack of potential for differentiation to that lineage in all conditions. (A) Analysis of a single-phenotypic HSC in vivo that may appear to be unipotent in primary transplants may reveal multipotent differentiation capacity in secondary transplants. (B) Analysis of progeny of the same HSC in vitro may demonstrate differentiation into single- or multiple-cell types, depending on the culture conditions. In this example, the single HSC studied is multipotent, a conclusion that can only be reached by multiple different lineage potential assays in combination. BFU-E, blast forming unit-erythroid; GM, granulocyte-monocyte; E, erythroid; MkE, megakaryocyte-erythroid; Mye, myeloid. Professional illustration by Patrick Lane, ScEYEnce Studios.