Illustration of the 2-compartment hidden stochastic model for hematopoiesis. The model designates 3 HSC behaviors: λ, the mean replication (self-renewal) rate (diagrammed as 1 HSC becoming 2 HSCs); α, the mean apoptosis rate (diagrammed as a HSC exiting from compartment 1); and ν, the mean rate at which an HSC becomes a multipotent progenitor (short-term repopulating) cell and gives rise to a differentiating clone. Once an HSC becomes a multipotent progenitor (diagrammed as an HSC entering compartment 2), it and its progeny (termed a differentiating clone) contribute to the blood cell production, which is observed for a mean of 1/μ weeks. With this tool, whether designated HSC behaviors could lead to experimental or clinical outcomes that are observed could be determined. See “Materials and methods” for further details. Adapted from Catlin et al.4 BFU-E, burst-forming unit erythroid; CFU, colony-forming unit; CFU-E, CFU erythroid; CFU-GM, CFU granulocyte macrophage; CFU-Meg, CFU megakaryocyte.