Opn^−/− mice show significant levels of endogenous mobilization and exhibit enhanced mobilization in response to G-CSF. (A) Opn^−/− mice exhibited significantly increased numbers of white blood cells (WBC) in the PB (B) and spleen compared with wild-type controls (P < .05). (C-D). This increase in WBC correlated with a significant increase in the percent, (E) number of HSC/HPC, (F) and frequency of CFC in the PB (P < .01). (G) Groups of 5 mice were mobilized with G-CSF for up to 6 days, before the analysis of the PB and spleen. (H) Opn^−/− mice exhibited significantly increased numbers of WBC, (I) and percent HSC/HPC in the PB 6 days after G-CSF. In addition, Opn^−/− mice exhibited significantly increased numbers of WBC (J) and percent HSC/HPC (K) in the spleen after 4 but not 6 days after G-CSF. *P < .05.