A method for increasing the cell surface expression of a TCR which comprises the steps of: (i) providing a TCR α chain and/or β chain sequence; (ii) determining an amino acid residue of the TCR α chain and/or β chain at one or more positions selected from: 96 of the α chain; 9 of the β chain; 10 of the β chain; 24 of the α chain; 19 of the α chain; and (iii) altering the amino acid residue at one or more of the positions listed in step (ii) to an amino acid residue selected from: leucine, isoleucine, valine or methionine at position 96 of the α chain; arginine, lysine or glutamine at position 9 of the β chain; tyrosine, phenylalanine, histidine or tryptophan at position 10 of the β chain; threonine or serine at position 24 of the α chain; valine, isoleucine, leucine or methionine at position 19 of the α chain; wherein the amino acid residues are numbered according to the International ImMunoGeneTics information system (IMGT), and with the proviso that when only one amino acid is altered in step (iii) the altered amino acid generated does not consist of an amino acid residue specified for the given position selected from group: L96 of the α chain; R9 of the β chain; Y10 of the β chain; T24 of the α chain; V19 of the α chain; and when two or more amino acid residues are altered in step (iii) at least one of the altered amino acid residues generated is not an amino acid residue specified for the given position selected from group. (i) providing a TCR α chain and/or β chain sequence; (ii) determining an amino acid residue of the TCR α chain and/or β chain at one or more positions selected from: 96 of the α chain; 9 of the β chain; 10 of the β chain; 24 of the α chain; 19 of the α chain; and (iii) altering the amino acid residue at one or more of the positions listed in step (ii) to an amino acid residue selected from: leucine, isoleucine, valine or methionine at position 96 of the α chain; arginine, lysine or glutamine at position 9 of the β chain; tyrosine, phenylalanine, histidine or tryptophan at position 10 of the β chain; threonine or serine at position 24 of the α chain; valine, isoleucine, leucine or methionine at position 19 of the α chain; wherein the amino acid residues are numbered according to the International ImMunoGeneTics information system (IMGT), and with the proviso that when only one amino acid is altered in step (iii) the altered amino acid generated does not consist of an amino acid residue specified for the given position selected from group: L96 of the α chain; R9 of the β chain; Y10 of the β chain; T24 of the α chain; V19 of the α chain; and when two or more amino acid residues are altered in step (iii) at least one of the altered amino acid residues generated is not an amino acid residue specified for the given position selected from group. leucine, isoleucine, valine or methionine at position 96 of the α chain; arginine, lysine or glutamine at position 9 of the β chain; tyrosine, phenylalanine, histidine or tryptophan at position 10 of the β chain; threonine or serine at position 24 of the α chain; valine, isoleucine, leucine or methionine at position 19 of the α chain; wherein the amino acid residues are numbered according to the International ImMunoGeneTics information system (IMGT), and with the proviso that when only one amino acid is altered in step (iii) the altered amino acid generated does not consist of an amino acid residue specified for the given position selected from group: L96 of the α chain; R9 of the β chain; Y10 of the β chain; T24 of the α chain; V19 of the α chain; and L96 of the α chain; R9 of the β chain; Y10 of the β chain; T24 of the α chain; V19 of the α chain; and when two or more amino acid residues are altered in step (iii) at least one of the altered amino acid residues generated is not an amino acid residue specified for the given position selected from group.