Non-human animals, and methods and compositions for making and using the same, are provided, wherein the non-human animals comprise a humanization of a Lymphocyte activation gene 3 (Lag3). The non-human animals may be described, in some embodiments, as having a genetic modification to an endogenous Lag3 locus so that the non-human animals express a Lag3 polypeptide that includes a human portion and an endogenous portion (e.g., a non-human portion).