NbALFA(3mut) is a single-domain antibody fragment, derived by mutagenesis of NbALFA, that binds the Tag:ALFA epitope. NbALFA(3mut) contains 3 amino acid substitutions (T75R, C100V and S124F) compared to NbALFA, resulting in it showing antigen-dependent stability; in the absence of Tag:ALFA the NbALFA(3mut) protein is partially destabilized, while in the presence of Tag:ALFA, the NbALFA(3mut) protein is stabilized and binds Tag:ALFA (FBrf0260618).