DLL - Fingers, Toes, Wingtips and Siphons
There is a homeobox gene called dll, which, if damaged, let Sean B. Carroll tell the story.
A few years ago, we were studying the Distal-less gene (Dll for short), so named because when it is mutated, the distal (outer) parts of fly limbs are lost. We were curious whether this limb-building gene played a role in other species. We were pleased to find that Dll was deployed in the distal parts of developing butterfly limbs, and in the limbs of crustaceans,
Carroll, Sean B.. Endless Forms Most Beautiful: The New Science of Evo Devo (p. 69). W. W. Norton & Company. Kindle Edition.
It happens that lobsters and fruit flies are not the only ones where it is active. All sorts of distal limb portions are under its control, including those appendages of mammals and the siphon of the sea squirt. Such homeobox genes appear to have a truly ancient heritage. The eyespots of primitive worms, the compound eyes of a fruit fly, and the camera eyes of mice and other mammals have their eye formation triggered by essentially the same gene. Transplant a bit of eye forming embryonic tissue from a mouse to a developing butterfly wing and it will develop a (butterfly) eye there.
One of the secrets of how evolution happens is that these master control genes can orchestrate formation of analogous but extremely different body parts in vastly different species.
Computer people - think reusable code.