HSC cycle and cell count A. (Left) In the absence of a CTA, Non-proliferating cells which are in the dormant phase (G0) are constantly signaled by microenvironmental cues to enter into the cycling state. Once signaled, the cells undergo mitosis. Cells exposed to an exogenous HIA are further induced to enter the (G1) phase, and undergo synthesis of DNA (S). The cells then enter the next phase (G2) where additional proteins involved in cell cycling are produced to allow for cell division to occur. Once the cells complete mitosis (M), they renter the resting phase. During this cycle, some cells propagate to differentiation paths (at a rate) into the various hematopoietic lines or lost by apoptosis (at a rate). However, in the presence of a strong CTA, the synthetic phase (S) is blocked, leading to apoptosis and loss of both differentiated and undifferentiated stem cells. (Right) HSC residing in the main sinus of the bone marrow (BM), divide into two new cells during the process of mitosis. During chemotherapy cell divisions are seriously compromised. B. Absolute CD34+ progenitor quantification in patient cohort receiving G-CSF (Neupogen). A total of 92 patients were assayed using flow cytometry and the ISHAGE protocol. Stem cell phenotype was confirmed through simultaneous detection of the CD34, CD38, and CD45 markers.