Zdk1 is a small engineered protein (based on the Z domain of immunoglobulin-binding staphylococcal protein A) that binds the dark state of the photoreactive flavin binding domain (LOV2) of the Avena sativa Phototropin 1 (PT1) gene with high affinity (binding requires both the LOV core domain and the C-terminal residues of the C-terminal Jα-helix of LOV2), but dissociates from LOV2 in the presence of blue light (PMID:27427858).