KOR was highly expressed in Flk1⁺ vascular progenitors. (A) RT-PCR showing mRNA expression of MOR, DOR, and KOR in ES cells, Flk1⁺ cells, cells after 1 or 3 days of Flk1⁺ cell culture (Flk-d1 or Flk-d3), CD31-positive cells (ECs) and CD31-negative cells (MCs) at Flk-d3. (B) Fluorescent staining for MOR, DOR, and KOR at Flk-d1. Nuclei are stained with DAPI (blue). Left, MOR; middle, DOR; right, KOR. Scale bars, 100 μm. (C) Double fluorescent staining for MOR, DOR, and KOR with CD31 (red) at Flk-d3. Nuclei are stained with DAPI (blue). Top, opioid (green) receptors (green); middle, CD31 (red); bottom, merged. Scale bars, 100 μm.