For the people asking questions about the experiment details, here is the full paper in Science (registration required): http://www.sciencemag.org/content/341/6

For the people asking questions about the experiment details, here is the full paper in Science (registration required): http://www.sciencemag.org/content/341/6143/275.full http://www.sciencemag.org/content/341/6143/275.full . Based on the paper, it appears that they performed targeted mutations to manipulate the Mrap2 gene and fed the mice equal quantities of chow, and then they did the tests described in the parent article. At least to me, this is strong evidence that this gene affects obesity in mice.