Specific expression of EPCR by SP cells. (A) RNA from sorted side population (SP) or main population (MP) cells was extracted and subjected to multiplex RT-PCR to detect expression of EPCR message. EPCR message was easily amplified from SP cell RNA but was nearly undetectable within a similar quantity of MP RNA (n = 3). (B) C57BL/6 total bone marrow was stained with Hoechst 33342 dye to identify the SP and further stained with mAb 1560, a monoclonal antibody specific for EPCR. Gray histograms denote level of staining produced by isotype control antibody. Red and blue histograms represent specific staining by mAb 1560. EPCR protein is readily detectable at the cell surface of SP cells with very little expression detectable in MP (n = 5).