Zdk2 is a small engineered protein (based on the Z domain of immunoglobulin-binding staphylococcal protein A) that specifically binds the dark state of the photoreactive LOV2 domain of the Avena sativa Phototropin 1 (PT1) gene. The binding of Zdk2 to the dark state of LOV2 does not require interaction with the C-terminal Jα helix of the LOV2 domain (PMID:27427858).