The matrix metalloproteinases have been associated with invasive and metastatic properties of many tumor types. However, the exact mechanism by which they influence these processes has remained elusive. Utilizing a novel orthotopic model of neuroblastoma, the investigators have used both genetic and pharmacologic approaches to this problem. The cover figure shows vascularization of a xenotransplanted neuroblastoma tumor visual-ized by fluorescent angiography using a FITC-labeled Lycopersicon escu-lentum lectin. The top figure shows the formation of an extensive capillary network in a wild-type mouse 3 weeks after tumor implantation. The bottom figure shows the defect in the vasculature in a mouse deficient in matrix metalloproteinase-9 (MMP-9). The data demonstrate a critical role for MMP-9 in blood vessel formation and, in particular, pericyte recruit-ment to the neovasculature. For details, see the article by Chantrain et al. on p. 1675 of this issue.