Flipping MLL1ʹs Switch One Proline at a Time

The MLL1 (mixed lineage leukemia 1) protein, which is often disrupted in leukemia, both activates and represses Hox genes during hematopoiesis. Now, demonstrate that the cyclophilin CyP33 underpins this regulatory switch by altering the state of MLL1 through cis-trans proline isomerization in the linker region between MLL1ʹs third PHD finger and bromodomain.