Studying microRNA function in vivo requires genetic strategies to generate loss-of-function phenotypes. We used lentiviral vectors to stably and specifically knock down microRNA by overexpressing microRNA target sequences from polymerase II promoters. These vectors effectively inhibited regulation of reporter constructs and natural microRNA targets. We used bone marrow reconstitution with hematopoietic stem cells stably overexpressing miR-223 target sequence to phenocopy the genetic miR-223 knockout mouse, indicating robust interference of microRNA function in vivo.