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The T-cell receptor sequences expressed on cells recognizing a specific peptide in the context of a given MHC molecule can be explored for common features that might explain their antigen specificity. However, despite the development of numerous experimental and bioinformatic strategies, the specificity problem remains unresolved. To address the need for additional experimental paradigms, we report here on an in vivo experimental strategy designed to artificially diversify a transgenic TCR by CRISPR/Cas9-mediated mutagenesis of Tcra and Tcrb chain genes. In this system, an initially monoclonal repertoire of known specificity is converted into an oligoclonal pool of TCRs of altered antigen reactivity. Tracking the fate of individual clonotypes during the intrathymic differentiation process illuminates the strong selective pressures that shape the repertoire of naïve T cells. Sequence analyses of the artificially diversified repertoires identify key amino acid residues in the CDR3 regions required for antigen recognition, indicating that artificial diversification of well-characterized TCR transgene sequences helps to reduce the complexities of learning the rules of antigen recognition., (© 2024 The Author(s). European Journal of Immunology published by Wiley‐VCH GmbH.)