VHH(GBP4) is a single-domain antibody fragment that binds the GFP protein. In vitro, binding of VHH(GBP4) to either GFP or EGFP leads to a reduction in their fluorescence by a factor of 5 and 8 fold respectively, and their spectral properties are modulated (in both cases, absorption at 475nm is reduced while absorption at 395nm is increased). These effects of VHH(GBP4)-binding on GFP and EGFP fluorescence and spectral properties are also seen in vivo. VHH(GBP4) corresponds to the antigen-binding variable domain (VHH domain) of a heavy chain antibody originally isolated from a camelid immunized with purified GFP protein (PMID:20010839).