To examine the possibility that biofluid exRNA may be a viable pharmacodynamic biomarker to monitor therapeutic drug target engagement in DMD patients, we examined urine RNA from an individual with an exon 45–50 deletion in the DMD gene who is being treated with eteplirsen, the exon-skipping ASO that was granted accelerated approval by the U.S. Food and Drug Administration (FDA) in September 201645. The gene discussed is DMD; the disease is Duchenne muscular dystrophy.