Down syndrome candidate region 1 (DSCR1) is one of more than 50 genes located in a region of chromosome 21 that has been implicated in Down syndrome. DSCR1 can be expressed as four isoforms, one of which, isoform 4 (DSCR1-4), has recently been found to be strongly induced by vascular endothelial growth factor A (VEGF-A¹⁶⁵) and to provide a negative feedback loop that inhibits VEGF-A¹⁶⁵-induced endothelial cell proliferation in vitro and angiogenesis in vivo. We report here that another DSCR1 isoform, DSCR1-1L, is also up-regulated by VEGF-A¹⁶⁵ in cultured endothelial cells and is strongly expressed in several types of pathologic angiogenesis in vivo. In contrast to DSCR1-4, the overexpression of DSCR1-1L induced the proliferation and activation of the transcription factor NFAT in cultured endothelial cells and promoted angiogenesis in Matrigel assays in vivo, even in the absence of VEGF-A. Similarly, small interfering RNAs specific for DSCR1-1L and DSCR1-4 had opposing inhibitory and stimulatory effects, respectively, on these same functions. DSCR1-4 is thought to inhibit angiogenesis by inactivating calcineurin, thereby preventing activation and nuclear translocation of NFAT, a key transcription factor. In contrast, DSCR1-1L, regulated by a different promoter than DSCR1-4, activates NFAT and its proangiogenic activity is inhibited by cyclosporin, an inhibitor of calcineurin. In sum, DSCR1-1L, unlike DSCR1-4, potently activates angiogenesis and could be an attractive target for antiangiogenesis therapy. (Mol Cancer Res 2006;4(11):811–20)