RNA interference is a recent, gene silencing technique that could be extremely valuable in studying gene function, treating diseases, and developing novel animal models for human diseases. Here, we investigated the feasibility of applying shRNA-mediated RNA interference in fetal fibroblasts for silencing of the myostatin gene and investigate adverse effects of RNAi We report that up to 97% silencing of myostatin mRNA was achieved using shRNA constructs in transiently and stably transfected fetal fibroblasts (p < 0.05). At the same time we also demonstrate that high level of shRNA expression resulted in 10- to 1000-fold induction of interferon responsive genes (OAS1, IFN-beta) (p < 0.05). In addition we also report novel adverse effect of shRNA expression in stably transfected cells-interference with microRNA processing/transport which led to 500-fold increase in the level of miR21 precursors (p < 0.05). Reduction of these side effects will be essential to obtain long term stable RNAi silencing. Crown Copyright (c) 2008 Published by Elsevier Inc. All rights reserved.