Interactions between tumour cells and endothelial cells play a crucial role in cancer progression and are particularly important for the generation of new blood vessels that supply tumours (tumour angiogenesis). The tumour cells are thought to produce factors that `activate' endothelial cells, promoting their proliferation and tube formation. Ralph Weichselbaum and co-workers have examined these interactions by generating expression profiles of human umbilical vein endothelial cells (HUVECs) co-cultured with U87 glioma cells(see p. 1013). They first demonstrate that the glioma cells induce an activated phenotype in the HUVECs(they proliferate and form net-like structures). The authors then use microarray analysis to reveal that the HUVECs are reprogrammed to express a variety of angiogenic receptor-ligand pairs (e.g. TGFβRII-TGFβ3,FGFRII-FGF7 and CCR5-RANTES), confirming their findings with quantitative PCR and immunohistochemical staining. They also show that conditioned media from the glioma cells can activate HUVECs cultured alone and that previously activated HUVECs can activate naive HUVECs that have not been exposed to tumour cells. The tumour cells must thus activate endothelial cells by releasing soluble factors that induce autocrine growth mechanisms.