An isolated protein comprising: (a) an immunoglobulin variable region comprising: (i) at least two cysteine residues positioned within framework region 2, wherein if at least two of the cysteine residues in FR2 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR2, for example, the disulphide bond is present under non-reducing conditions; and/or (ii) at least two cysteine residues positioned within framework region 3, wherein if at least two of the cysteine residues in FR3 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR3, for example, the disulphide bond is present under non-reducing conditions; or (b) an immunoglobulin heavy chain variable region (V H) and an immunoglobulin light chain variable region (V L), wherein at least one of the variable regions comprises: (i) at least two cysteine residues positioned within framework region 2, wherein if at least two of the cysteine residues in FR2 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR2, for example, the disulphide bond is present under non-reducing conditions; and/or (ii) at least two cysteine residues positioned within framework region 3, wherein if at least two of the cysteine residues in FR3 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR3, for example, the disulphide bond is present under non-reducing conditions, wherein the protein comprises a compound conjugated to at least one of the cysteine residues, and wherein conjugation of the compound does not prevent binding of the protein to an antigen. (a) an immunoglobulin variable region comprising: (i) at least two cysteine residues positioned within framework region 2, wherein if at least two of the cysteine residues in FR2 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR2, for example, the disulphide bond is present under non-reducing conditions; and/or (ii) at least two cysteine residues positioned within framework region 3, wherein if at least two of the cysteine residues in FR3 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR3, for example, the disulphide bond is present under non-reducing conditions; or (i) at least two cysteine residues positioned within framework region 2, wherein if at least two of the cysteine residues in FR2 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR2, for example, the disulphide bond is present under non-reducing conditions; and/or (ii) at least two cysteine residues positioned within framework region 3, wherein if at least two of the cysteine residues in FR3 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR3, for example, the disulphide bond is present under non-reducing conditions; or (b) an immunoglobulin heavy chain variable region (V H) and an immunoglobulin light chain variable region (V L), wherein at least one of the variable regions comprises: (i) at least two cysteine residues positioned within framework region 2, wherein if at least two of the cysteine residues in FR2 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR2, for example, the disulphide bond is present under non-reducing conditions; and/or (ii) at least two cysteine residues positioned within framework region 3, wherein if at least two of the cysteine residues in FR3 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR3, for example, the disulphide bond is present under non-reducing conditions, (i) at least two cysteine residues positioned within framework region 2, wherein if at least two of the cysteine residues in FR2 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR2, for example, the disulphide bond is present under non-reducing conditions; and/or (ii) at least two cysteine residues positioned within framework region 3, wherein if at least two of the cysteine residues in FR3 are not conjugated to a compound then an intra-framework region disulphide bond is capable of forming between the cysteine residues in FR3, for example, the disulphide bond is present under non-reducing conditions,