Down Syndrome (DS) is a genetic disorder caused by full or partial trisomy of chromosome 21. It occurs in approximately 1/750 live births and presents with many clinical phenotypes including a reduced incidence of solid tumours1,2. Recent work using the Ts65Dn model of DS, that has orthologs of approximately 50% of the genes on human chromosome 21 (Hsa21), has suggested that three copies of the ETS23 or Down Syndrome candidate region 1 (DSCR1) genes4 (a previously known suppressor of angiogenesis5,6) is sufficient to inhibit tumour growth. We have used the Tc1 transchromosomic mouse model of DS9 to dissect the contribution of extra copies of genes on Hsa21 to tumour angiogenesis. This mouse expresses approximately 81% of Hsa21 genes but not the human DSCR1 region (Supplementary Fig. 1). We transplanted B16F0 and Lewis Lung Carcinoma (LLC) tumour cells into Tc1 mice and showed that growth of these tumours was reduced substantially when compared to wild-type littermate controls. Furthermore, tumour angiogenesis was repressed significantly in Tc1 mice. In particular, in vitro and in vivo angiogenic responses to vascular endothelial growth factor (VEGF) were inhibited. Examination of the genes on the segment of Hsa21 in Tc1 mice identified putative anti-angiogenic genes (ADAMTS17,8 and ERG9) and novel endothelial cell-specific genes10, never shown before to be involved in angiogenesis (JAM-B11 and PTTG1IP) that, when overexpressed, are responsible for the inhibition of angiogenic responses to VEGF. Three copies of these genes within the stromal compartment reduced tumour angiogenesis providing an explanation for the reduced tumour growth in DS. Furthermore, we anticipate that, in addition to the candidate genes that we show to be involved in the repression of angiogenesis, the Tc1 mouse model of DS will likely allow for the identification of other endothelial-specific anti-angiogenic targets relevant to a broad spectrum of cancer patients.