Background/Aims: ALT1 is a novel long non-coding RNA derived from the alternatively spliced transcript of the deleted in lymphocytic leukemia 2 (DLEU2). To date, ALT1 biological roles in human vascular endothelial cells have not been reported. Methods: ALT1 was knocked down by siRNAs. Cell proliferation was analyzed by cck-8. The existence and sequence of human ALT1 were identified by 3' rapid amplification of cDNA ends. The interaction between lncRNA and proteins was analyzed by RNA-Protein pull down assay, RNA immunoprecipitation, and mass spectrometry analysis. Results: ALT1 was expressed in human umbilical vein endothelial cells (HUVECs). The expression of ALT1 was significantly downregulated in contact-inhibited HUVECs and in hypoxia-induced, growth-arrested HUVECs. Knocking down of ALT1 inhibited the proliferation of HUVECs by G0/G1 cell cycle arrest. We observed that angiotensin converting enzyme â...i(ACE2) was a direct target gene of ALT1. Knocking-down of ALT1 or its target gene ACE2 could efficiently decrease the expression of cyclin D1 via the enhanced ubiquitination and degradation, in which HIF-1α and protein von Hippel-Lindau (pVHL) might be involved. Conclusion: The results suggested the human long non-coding RNA ALT1 is a novel regulator for cell cycle of HUVECs via ACE2 and cyclin D1 pathway.