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Purpose of review: We highlight some recent high-profile articles in which the study results and interpretations, upon comparison, convey the controversy and disparate views prevalent in the field when defining the role of murine bone marrow-derived endothelial progenitor cells in the process of tumor neoangiogenesis.

Recent findings: Highlighted articles identify a critical role for these cells in promoting the angiogenic switch for growth of metastatic tumors or find no role at all for these circulating cells as engrafting cells in vascular endothelium or as promoters of tumor growth. We suggest potential strategies to further document cell identities as a guide for future studies and provide some alternative interpretations of the published studies highlighted in this review that, if considered, may diminish the current polar views.

Summary: To fully appreciate the orchestrated cellular events participating in new vessel formation in the tumor microenvironment, better methods of cell identification and functional validation will be necessary to specify which cells in which tumors are facilitating and forming new vessels in which tissues at which stage of the disease.

(C) 2009 Lippincott Williams & Wilkins, Inc.