CXCR4 and placenta praevia: In the meantime, CXCL12, CXCR4, and CXCR7 mRNA levels in placental tissues were also evaluated by RT-qPCR assay, which elucidated that high CXCL12 and its receptor CXCR4 and CXCR7 levels were witnessed in the placental tissues of patients with placenta previa in comparison to those in the placental tissues of normal puerperae (Figure 1c).