The present invention relates to a soluble negative control MHC multimer comprising a nonsense peptide that binds the MHC protein efficiently, but that does not support binding of the resultant MHC-peptide complex to the desired T Cell Receptor. The nonsense peptide is designed to i) have a length enabling binding to the MHC allele in question, ii) have appropriate amino acids at relevant anchor positions which anchor the nonsense peptide to the peptide-binding groove of the MHC, iii) have amino acids outside the anchor positions that do not support binding to a T Cell Receptor, and iv) have an amino acid sequence that is different from the linear sequence of any naturally occurring peptide.
