A few days ago I attended a lecture about the protein-degredation mechanism in cells. It's pretty complicated, but basically this is what happens: an enzyme complex called a ubiquitin ligase recognizes proteins that should be degraded and attaches a chain of ubiquitin moledules to it. This chain of ubiquitin is then recognized by a special protein structure which chews up the marked protein, releasing the amino acids.

A certain class of ubiquitin ligases is roughly U-shaped, with the ubiquitin on one end and a domain to bind the target protein on the other. Binding the target brings it close enough to the opposite arm to ubiquinate it. The ubiquitin ligase needs to be able to recognize many different proteins, and it is able to do this simply by switching out the protein (called an F-box adapter) at the binding end of the "U".

One of the slides illustrating this showed several ubiquitin ligases, each with a different F-box adapter represented by geometric shapes, and the proteins that particular adapter could ubiquinate. As I was looking at that slide, I had a strange moment of recognition: I've seen that before, I thought. Holy Crap! Is that a design pattern? Yes, it is. It turns out that ubiquitin ligase implements the strategy design pattern, made famous in computer science by the book Design Patterns.