An in vitro method for the identification of a TCR recognizable peptide sequence, comprising: i. contacting a plurality of mammalian cells, wherein - each of said plurality of mammalian cells expresses a member of a library; - each member of said library encodes a transgenic antigen receptor; - said transgenic antigen receptor comprises: • an extracellular domain of MHC1 or MHC2, • an oligopeptide comprised within said extracellular domain, wherein the oligopeptide is different for each member of the library, and wherein the oligopeptide is presented in a way suitable for the recognition by a T cell receptor, • a transmembrane domain of a T-cell receptor (TCR), and • an intracellular domain of a TCR; - said transgenic antigen receptor is functionally linked to a reporter gene encoding a reporter protein, whereby binding of a cognate T cell receptor to said transgenic antigen receptor results in the activation of said reporter gene, with a preparation of T-lymphocytes, ii. separating cells showing a detectable reporter protein from said plurality of mammalian cells according to the detectable level of said reporter protein, yielding activated cells, iii. isolating DNA from said activated cells, and iv. sequencing of the isolated DNA encoding said oligopeptide sequence comprised in said transgenic antigen-receptor. i. contacting a plurality of mammalian cells, wherein - each of said plurality of mammalian cells expresses a member of a library; - each member of said library encodes a transgenic antigen receptor; - said transgenic antigen receptor comprises: • an extracellular domain of MHC1 or MHC2, • an oligopeptide comprised within said extracellular domain, wherein the oligopeptide is different for each member of the library, and wherein the oligopeptide is presented in a way suitable for the recognition by a T cell receptor, • a transmembrane domain of a T-cell receptor (TCR), and • an intracellular domain of a TCR; - said transgenic antigen receptor is functionally linked to a reporter gene encoding a reporter protein, whereby binding of a cognate T cell receptor to said transgenic antigen receptor results in the activation of said reporter gene, with a preparation of T-lymphocytes, - each of said plurality of mammalian cells expresses a member of a library; - each member of said library encodes a transgenic antigen receptor; - said transgenic antigen receptor comprises: • an extracellular domain of MHC1 or MHC2, • an oligopeptide comprised within said extracellular domain, wherein the oligopeptide is different for each member of the library, and wherein the oligopeptide is presented in a way suitable for the recognition by a T cell receptor, • a transmembrane domain of a T-cell receptor (TCR), and • an intracellular domain of a TCR; • an extracellular domain of MHC1 or MHC2, • an oligopeptide comprised within said extracellular domain, wherein the oligopeptide is different for each member of the library, and wherein the oligopeptide is presented in a way suitable for the recognition by a T cell receptor, • a transmembrane domain of a T-cell receptor (TCR), and • an intracellular domain of a TCR; - said transgenic antigen receptor is functionally linked to a reporter gene encoding a reporter protein, whereby binding of a cognate T cell receptor to said transgenic antigen receptor results in the activation of said reporter gene, with a preparation of T-lymphocytes, ii. separating cells showing a detectable reporter protein from said plurality of mammalian cells according to the detectable level of said reporter protein, yielding activated cells, iii. isolating DNA from said activated cells, and iv. sequencing of the isolated DNA encoding said oligopeptide sequence comprised in said transgenic antigen-receptor. <EOS>