Dogs, interestingly enough, show greater size variation than any other terrestrial mammal. Last week in Science, Sutter et al. reported finding the locus controlling size variation among dogs. A single allele (with a single nucleotide difference from large dogs) at the IGF1 locus is found in all small dogs (but probably not Sarah, at right). IGF1 produces an insulin-like growth factor and is linked to size mice and humans. Presumably, if you knock out IGF1 function, you will get a dog no bigger than a large rodent. Although Sutter et al. didn't speculate as such, I will boldly predict that this locus is pleiotropic (i.e. one gene, many functions). My guess is that IGF1 also functions as a Yap repressor. In the absence of IGF1, the Yap pathway is active and much yapping ensues. For want of a Yap repressor, those very small friends of ours have an unfortunate tendency to yap incessantly.