nox-4 activity resulted in decreased hydrogen peroxide production and decreased production of oxidant inducible monocyte chemoattractant protein-1. ConClusions:These are the first reported results to demonstrate the significant contribution of dicer activity and miR production in promoting hemangioendothelioma growth in vivo. These are also the first reported results of miR21a-3p targeting nox-4 mRNA and inhibiting reactive oxygen species production in endothelial cells. Collectively, these results indicate that targeting microRNA and specifically, miR-21a-3p, represent potential therapeutic strategies for the treatment of endothelial cell tumors including hemangioma and hemangioendothelioma.