VHH(GBP1) is a single-domain antibody fragment that binds the GFP fluorescent protein and its derivative EGFP. In vitro, binding of VHH(GBP1) to either GFP or EGFP leads to an enhancement of their fluorescence by a factor of fourfold and ~1.5 fold respectively, and their spectral properties are modulated (in both cases, absorption at 475nm is increased while absorption at 395nm is decreased). These effects of VHH(GBP1)-binding on GFP and EGFP fluorescence and spectral properties are also seen in vivo. VHH(GBP1) 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).