A variant chimeric receptor comprising a modified junction region having one or more amino acid sequence modifications compared to a junction region of a reference chimeric receptor, wherein: the reference chimeric receptor comprises a CD28 transmembrane domain and a 4-1BB signaling domain, joined in contiguous sequence at a junction, wherein the junction region of the reference chimeric receptor comprises up to 15 contiguous amino acids directly C-terminal of the junction and/or up to 15 contiguous amino acids directly N-terminal of the junction; and a peptide fragment having the sequence of an 8-15 amino acid portion of the modified junction region and having a binding affinity for a human leukocyte antigen molecule that is lower than the binding affinity, for the same HLA molecule, of a peptide fragment having the sequence of the corresponding portion of the junction region of the reference chimeric receptor, wherein: the one or more modifications comprises an amino acid replacement(s) and the amino acid replacement(s) are at one or more residues corresponding to a residue selected from 28, 31 and 34, wherein the amino acid replacement(s) corresponds to or is a replacement selected from among K28A, K28H, K28L, K28Q, K28S, R31A, R31H, R31L, R31N, R31S, L34A and L34S, with reference to numbering set forth in SEQ ID NO:5, and wherein: the variant chimeric receptor comprises a modified junction region comprising less than 100% sequence identity to SEQ ID NO:137 but greater than 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95% or 96% sequence identity to SEQ ID NO:137 and comprises the modifications; and/or the variant chimeric receptor comprises a sequence comprising less than 100% sequence identity to SEQ ID NO:5 but greater than 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95% or 96% sequence identity to SEQ ID NO:5 and comprises the modifications. the reference chimeric receptor comprises a CD28 transmembrane domain and a 4-1BB signaling domain, joined in contiguous sequence at a junction, wherein the junction region of the reference chimeric receptor comprises up to 15 contiguous amino acids directly C-terminal of the junction and/or up to 15 contiguous amino acids directly N-terminal of the junction; and a peptide fragment having the sequence of an 8-15 amino acid portion of the modified junction region and having a binding affinity for a human leukocyte antigen molecule that is lower than the binding affinity, for the same HLA molecule, of a peptide fragment having the sequence of the corresponding portion of the junction region of the reference chimeric receptor, wherein: the one or more modifications comprises an amino acid replacement(s) and the amino acid replacement(s) are at one or more residues corresponding to a residue selected from 28, 31 and 34, wherein the amino acid replacement(s) corresponds to or is a replacement selected from among K28A, K28H, K28L, K28Q, K28S, R31A, R31H, R31L, R31N, R31S, L34A and L34S, with reference to numbering set forth in SEQ ID NO:5, and wherein: the variant chimeric receptor comprises a modified junction region comprising less than 100% sequence identity to SEQ ID NO:137 but greater than 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95% or 96% sequence identity to SEQ ID NO:137 and comprises the modifications; and/or the variant chimeric receptor comprises a sequence comprising less than 100% sequence identity to SEQ ID NO:5 but greater than 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95% or 96% sequence identity to SEQ ID NO:5 and comprises the modifications. the variant chimeric receptor comprises a modified junction region comprising less than 100% sequence identity to SEQ ID NO:137 but greater than 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95% or 96% sequence identity to SEQ ID NO:137 and comprises the modifications; and/or the variant chimeric receptor comprises a sequence comprising less than 100% sequence identity to SEQ ID NO:5 but greater than 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95% or 96% sequence identity to SEQ ID NO:5 and comprises the modifications.