NbALFA(5mut) is a single-domain antibody fragment, derived by mutagenesis of NbALFA, that binds the Tag:ALFA epitope. NbALFA(5mut) contains 5 amino acid substitutions (T25V, T75R, C100V, Q116H and S124F) compared to NbALFA, resulting in it showing antigen-dependent stability; in the absence of Tag:ALFA the NbALFA(5mut) protein is largely unstable, while in the presence of Tag:ALFA, the NbALFA(5mut) protein is stabilized and binds Tag:ALFA (FBrf0260618).