Detection of CXCR4 transcripts by RNase protection assay. mRNA was extracted from peripheral blood mononuclear cells, PHA-activated mononuclear cells, CD41⁺ cells, and platelets. tRNA served as a negative control. Samples of mRNA were hybridized with a specific CXCR4 riboprobe and a specific actin riboprobe, then digested with RNase T1. Protected fragments were analyzed on denaturing acrylamide gels. Sizes of the fragments were determined using labeled Msp I–digested pBR322.