After some first  experiments in human embryos, there is a new Chinese paper in the Journal of Molecular Biology showing that also gene doping is possible in mammals.  Myostatin deficiency otherwise leads to some really impressive super strength children while it is now possible to knockout this gene artificially.  Hopefully the WADA will test for myostatin gene activity in Rio 2016!
Addendum 4 Dec15: An International Summit Statement On Human Gene Editing says

It would be irresponsible to proceed with any clinical use of germline editing unless and until (i) the relevant safety and efficacy issues have been resolved, based on appropriate understanding and balancing of risks, potential benefits, and alternatives, and (ii) there is broad societal consensus about the appropriateness of the proposed application.

Both conditions are unlikely to be ever met.