Scientists have been experimenting with ways to tackle problems like the muscle loss that occurs with aging. They injected aging mice with a gene that promotes muscle growth and found that they didn't lose muscle. When young mice were injected with the gene, they grew abnormally muscular.
If a young, naturally athletic person were to be injected with a muscle building gene it's possible they could become super strong athletes able to hit hundreds of home runs.
Gene doping hasn't been tried on humans yet, and we don't know anything about potential side effects. Maybe scientists will develop some test to detect artificial proteins. However, given the fame and fortune to be made from sports, it's a pretty sure bet that at some point athletes will start injecting genes.