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takyon writes:

Researchers from Seoul National University have created piglets with abnormal muscle growth by disrupting a gene that inhibits muscle cell growth:

Key to creating the double-muscled pigs is a mutation in the myostatin gene (MSTN). MSTN inhibits the growth of muscle cells, keeping muscle size in check. But in some cattle, dogs and humans, MSTN is disrupted and the muscle cells proliferate, creating an abnormal bulk of muscle fibres. To introduce this mutation in pigs, Kim used a gene-editing technology called a TALEN, which consists of a DNA-cutting enzyme attached to a DNA-binding protein. The protein guides the cutting enzyme to a specific gene inside cells, in this case in MSTN, which it then cuts. The cell's natural repair system stitches the DNA back together, but some base pairs are often deleted or added in the process, rendering the gene dysfunctional.

The team edited pig fetal cells. After selecting one edited cell in which TALEN had knocked out both copies of the MSTN gene, Kim's collaborator Xi-jun Yin, an animal-cloning researcher at Yanbian University in Yanji, China, transferred it to an egg cell, and created 32 cloned piglets. Kim and his team have not yet published their results. However, photographs of the pigs "show the typical phenotype" of double-muscled animals, says Heiner Niemann, a pioneer in the use of gene-editing tools in pigs who is at the Friedrich Loeffler Institute in Neustadt, Germany. In particular, he notes, they have the pronounced rear muscles that are typical of such animals. Yin says that preliminary investigations, show that the pigs provide many of the double-muscled cow's benefits — such as leaner meat and a higher yield of meat per animal. However, they also share some of its problems. Birthing difficulties result from the piglets' large size, for instance. And only 13 of the 32 lived to 8 months old. Of these, two are still alive, says Yin, and only one is considered healthy. Rather than trying to create meat from such pigs, Kim and Yin plan to use them to supply sperm that would be sold to farmers for breeding with normal pigs. The resulting offspring, with one disrupted MSTN gene and one normal one, would be healthier, albeit less muscly, they say; the team is now doing the same experiment with another, newer gene-editing technology called CRISPR/Cas9. Last September, researchers reported using a different method of gene editing to develop new breeds of double-muscled cows and double-muscled sheep (C. Proudfoot et al. Transg. Res. 24, 147–153; 2015).

A mutation in MSTN could occur naturally, and no gene transfer is involved. No genetically engineered animal has been approved for human consumption by any of the world's regulators, but the U.S. and Germany have passed on regulating gene-edited crops that do not incorporate new DNA in the genome.