.....In order to characterize the role of miRNAs in cellular senescence of endothelial cells, we performed miRNA arrays from HUVECs of 5 different donors. Twelve miRNAs, comprising hsa-miR-23a, hsa-miR-23b, hsa-miR-24, hsa-miR-27a, hsa-miR-29a, hsa-miR-31, hsa-miR-100, hsa-miR-193a, hsa-miR-221, hsa-miR-222 and hsa-let-7i are consistently up-regulated in replicatively senescent cells. Surprisingly, also miR-21 was found up-regulated by replicative and stress induced senescence, despite being described as oncogenic. Transfection of early passage endothelial cells with miR-21 resulted in lower angiogenesis, and less cell proliferation mirrored by up-regulation of p21CIP 1 and down-regulation of CDK2. These two cell cycle regulators are indirectly regulated by miR-21 via its validated direct targets NFIB (Nuclear factor 1 B-type), a transcriptional inhibitor of p21CIP 1 , and CDC25A, which regulates CDK2 activity by dephosphorylation. Knock-down of either NFIB or CDC25A show a phenocopy of over-expressing miR-21 in regard to cell cycle arrest. Finally, miR-21 overepxression reduces the replicative life span, while stable knock-down by sponges extends the replicative life span of endothelial cells. Therefore, we propose that miR-21 is the first miRNA that upon its knock-down extends the replicative life span of normal human cells.