These are two molecular models of a lambda bacteriophage Cro protein and a region of DNA to which it binds. As long as this protein is bound to DNA, the phage will remain dormant. Stress triggers degradation of this protein and re-expression of the phage.

The narrow towers don't quite print right on my bot, your mileage may vary. I wish I'd made them every so slightly smaller so they fit better, or had some sort of sand-blasting polishing tool.