CXCL12/CXCR4 controls the circadian rhythm of leukocyte trafficking. Blood samples were taken from 10-week-old NSG and humanized mice every 6 hours (ZT1-ZT25). (A) Mouse and human CD45⁺ leukocytes were analyzed for CXCR4 expression by RT-PCR. Shown are fold changes of CXCR4 mRNA levels after normalizing data from each point in individual mice to the initial time ZT1 (n = 6). (B-E) Blood samples were analyzed by flow cytometry for the expression of CXCR4 at ZT7 and ZT19 (n = 6). Shown are representative flow cytometry plots and statistical analysis. (F-I) Humanized mice were treated with PBS and antibodies against CXCL12 (F-G) (n = 6) or CXCR4 (H-I) (n = 6), followed by sampling of blood from ZT1 to ZT25. The cell counts at each point in individual mice were normalized to the initial cell count at point ZT1. Shown are fold changes of numbers of mCD45⁺ cells and hCD45⁺ cells. *P < .05; **P < .01.