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