(A) T lymphocytes are normally in the resting, or G₀, phase of the cell cycle. Signals from the TCR promote the transition from G₀ to G₁, which renders the cells competent to progress further through the cell cycle by stimulating the expression of the genes encoding IL-2 and the subunits of IL-2R. Subsequently, the IL-2–IL-2R interaction is responsible for blastic transformation and G₁ progression, which moves the cell through the G₁ phase of the cell cycle to the point that the cells no longer require signals from the IL-2–IL-2R interaction (i.e., past the R-point [R]) to enter S phase and subsequently undergo mitosis. (B) HSCs, like resting mature T lymphocytes, are also normally in G₀. Unlike the T lymphocyte lineage, little is known regarding the cytokines and receptors responsible for stimulating the HSCs to leave G₀ and enter the cell cycle. Even so, once this G₀-G₁ transition occurs and an HSC has given rise to a committed progenitor cell, such as a committed granulocyte precursor, the cytokines responsible for G₁ progression are known, and this is exemplified in the figure by G-CSF acting on a committed granulocyte precursor to promote the progression from the G₁ to the S, G₂, and M phases.