From ScienceDaily: scientists "have tamed a virus so that it attacks and destroys cancer cells but does not harm healthy cells. They determined how to produce replication-competent viruses with key toxicities removed ... Cellular microRNA molecules regulate the stability of [messenger RNA] in different cell types, and this newly-understood mechanism provides the possibility to engineer viruses for cell-specific inactivation. [This is] a mechanism whereby wild type virus potency could be maintained in tumor cells but the virus could be 'turned off' in tissues vulnerable to pathology. ... This approach is surprisingly effective and quite versatile. It could find a range of applications in controlling the activity of therapeutic viruses, both for cancer research and also to engineer a new generation of conditionally-replicating vaccines. ... Although the current tumor-killing virus is useful in mice, transfer of the technology into the clinical setting will require re-engineering of the virus to overcome virus pathologies seen in humans, and it will be at least two years before this can be tested in the clinics."