VHH(GFP4) is a single-domain antibody fragment that binds the GFP fluorescent protein and some of its derivatives, including YFP, EYFP and Venus. It does not bind CFP or any derivatives of the Discosoma drFP583 gene (represented in FlyBase by Disc\RFP). VHH(GFP4) corresponds to the antigen-binding variable domain (VHH domain) of a heavy chain antibody originally isolated from an alpaca (Lama pacos) immunized with purified GFP protein (PMID:17951627, PMID:17060912, PMID:16095608, FBrf0233758).