It has been assumed that endothelial cells associated with tumors are genetically stable diploid cells, unlike tumor cells, which are unstable and can become drug resistant. The stability of tumor endothelial cells has made these cells attractive as targets for antitumor angiogenesis therapy. However, we have isolated mouse endothelial cells that have invaded a human tumor xenograft and, using FISH (fluorescent in situ hybridization) analysis, have found that they unexpectedly contain a population with cytogenetic abnormalities. In the top image, FISH analysis of freshly isolated noncultured endothelial cells (green color) detects multiple chromosomes (red signals). In the bottom image, multicolor FISH analysis of cultured tumor endothelial cells detects chromosomal aberrations, including multiple chromosomes, translocations, and deletions. In contrast, normal endothelial cell counterparts are diploid to begin with and remain so even after long-term culture. These results indicate that tumor endothelial cells differ cytogenetically from normal ones and offer new and significant insights into tumor angiogenesis. For details, see the articles in this issue by Hida et al. on page 8249 of this issue.

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