Expression of S1P receptor mRNAs in CD34⁺ hematopoietic progenitors. (A) RT-PCR analysis of S1P₁, S1P₂, S1P₃, S1P₄, S1P₅, and β-actin expression was carried out with RNA isolated from mobilized PB CD34⁺ cells originating from 4 different donors as described in “Materials and methods.” CD56⁺ cells and HUVECs served as positive controls for S1PR expression. Control experiments in the absence of RT are also shown. All samples of PB CD34⁺ progenitors highly and constantly expressed mRNA for S1P₁, the receptor with the highest affinity for FTY720 (in this figure, S1PRs are arranged according to the affinity for FTY720 from high to low). (B) The expression pattern of S1PRs in FACS-sorted CD34⁺/CD38- and CD34⁺/CD38⁺ progenitors was compared. While S1P₁ was expressed in both subsets, S1P₃ and S1P₅ were positive only in CD34⁺/CD38^- progenitors. In contrast, S1P₂ and S1P₄ mRNA was found only in CD34⁺/CD38⁺ cells.