(A) Basal levels of HSC egress from the BM into the PB sinusoid. (B) Stimulated release of HSCs from the BM into the PB facilitated by the S1P/S1P1 axis gradient after disruption of the SDF-1/CXCR4 axis that normally retains HSCs in the bone marrow. (C) Increased levels of HSC mobilization into the PB achieved through augmentation of S1P in the PB in conjunction with disruption of SDF-1/CXCR4 axis. Professional illustration by Paulette Dennis.