Sonic hedgehog is at work in mice limb buds in what is known as the ectoderm, the cell layer that gives rise to skin, researchers discovered. Finding Sonic hedgehog here is akin to discovering that yeast has crept from the batter to the frosting, where it has the surprising effect of limiting how much the cake rises. In this case, instead of causing appendages to grow in mice, Sonic hedgehog prevents digits from developing.