G-CSF treatment does not increase the number of circulating progenitors in CXCR4^−/− chimeras. Wild-type (CXCR4^+/+) and CXCR4^−/− chimeras (n = 7-10 each group) were treated with G-CSF or left untreated (control), and CFU-Cs were measured in (A) peripheral blood, (B) spleen, and (C) bone marrow. Data represent mean ± SEM of pooled data from 2 separate experiments; *P < .01.