AC-133 immunocytochemistry of circulating endothelial cells. Note that circulating endothelial cells from a patient after allogeneic hematopoietic stem cell transplantation were AC-133 negative (A-C), whereas human endothelial progenitor cells served as positive controls (panel D; donated by Dr F.H. Bahlmann); we used murine anti-human AC-133 antibody (Miltenyi), and APAAP technique (Dako).