A method for predicting whether an immunotherapeutic TCR has the potential to cross react with another peptide in the subject in vivo to give rise to unwanted side effects derived from off target specificities in said subject, wherein the TCR binds to a target peptide presented by a Major Histocompatibility Complex and is for administration to a subject; the method comprising: (a) identifying at least one binding motif in the target peptide to which the TCR binds; (b) searching for peptides that are present in the subject that comprise the at least one binding motif and that are not the target peptide; and (c) testing binding of any such peptide to the TCR in vitro, (d) wherein the presence of one or more such peptides that bind the TCR indicates that the TCR has the potential to cross react in vivo and cause unwanted side effects; the at least one binding motif being identified by: (i) creating a series of mutants of the target peptide, each mutant having the amino acid residue at one position in the binding sequence thereof that is involved in binding to the TCR substituted for alanine or glycine, such that over the series of mutants the amino acid residue in each position in the binding sequence is substituted for alanine or glycine; and testing each mutant in the series for its activity relative to the wild type target peptide, wherein an amino acid residue at a position within the binding sequence is identified as being part of the binding motif if the mutant in which the amino acid at that position is mutated to alanine or glycine has a 50% or greater loss of activity relative to the wild type target peptide, or (ii) creating a series of mutants, each mutant having the amino acid residue at one position in the binding sequence substituted for an alternative amino acid, such that over the series of mutants the amino acid residue in each position in the binding sequence is substituted for all alternative amino acids; and testing each mutant in the series for its activity relative to the wild type peptide, wherein amino acid substitutions which result in a 50% or greater loss of activity relative to the wild type target peptide are considered to be non-tolerated amino acids and not part of the binding motif and amino acids substitutions which do not result in a 50% or greater loss of activity relative to the wild type target peptide are considered as part of the binding motif. the method comprising: (a) identifying at least one binding motif in the target peptide to which the TCR binds; (b) searching for peptides that are present in the subject that comprise the at least one binding motif and that are not the target peptide; and (c) testing binding of any such peptide to the TCR in vitro, (d) wherein the presence of one or more such peptides that bind the TCR indicates that the TCR has the potential to cross react in vivo and cause unwanted side effects; (a) identifying at least one binding motif in the target peptide to which the TCR binds; (b) searching for peptides that are present in the subject that comprise the at least one binding motif and that are not the target peptide; and (c) testing binding of any such peptide to the TCR in vitro, (d) wherein the presence of one or more such peptides that bind the TCR indicates that the TCR has the potential to cross react in vivo and cause unwanted side effects; the at least one binding motif being identified by: (i) creating a series of mutants of the target peptide, each mutant having the amino acid residue at one position in the binding sequence thereof that is involved in binding to the TCR substituted for alanine or glycine, such that over the series of mutants the amino acid residue in each position in the binding sequence is substituted for alanine or glycine; and (i) creating a series of mutants of the target peptide, each mutant having the amino acid residue at one position in the binding sequence thereof that is involved in binding to the TCR substituted for alanine or glycine, such that over the series of mutants the amino acid residue in each position in the binding sequence is substituted for alanine or glycine; and testing each mutant in the series for its activity relative to the wild type target peptide, wherein an amino acid residue at a position within the binding sequence is identified as being part of the binding motif if the mutant in which the amino acid at that position is mutated to alanine or glycine has a 50% or greater loss of activity relative to the wild type target peptide, or (ii) creating a series of mutants, each mutant having the amino acid residue at one position in the binding sequence substituted for an alternative amino acid, such that over the series of mutants the amino acid residue in each position in the binding sequence is substituted for all alternative amino acids; and testing each mutant in the series for its activity relative to the wild type peptide, wherein amino acid substitutions which result in a 50% or greater loss of activity relative to the wild type target peptide are considered to be non-tolerated amino acids and not part of the binding motif and amino acids substitutions which do not result in a 50% or greater loss of activity relative to the wild type target peptide are considered as part of the binding motif.