Animals with IL-15 Humanized

A genetically modified mouse whose genome comprises a substitution at an endogenous locus of IL-15 mouse, a genomic fragment of mouse encoding a mature IL-15 mouse with a human genomic segment encoding a mature polypeptide human IL-15 wherein the exons of human IL-15 in said human genomic segment consisting of exons 3, 4, 5 and 6 encoding the human IL-15 protein, to form an IL-15 humanized, where exons of IL-15 are listed with the first coding exon and exon 1, and wherein the IL-15 humanized is under the control of all current endogenous regulatory elements above IL-15 mouse locus endogenous mouse IL-15.