The present invention relates to a genetically modified rodent comprising in its germline a replacement of rodent IL-7 exons with human IL-7 exons 2 through 6 to form a humanized IL-7 gene, wherein the replacement is at an endogenous rodent IL-7 locus and the resulting modified rodent IL-7 locus lacks rodent animal IL-7 exons 2, 3, 4 and 5, and wherein the humanized IL-7 gene is under control of endogenous rodent IL-7 5’ regulatory elements. The present invention also relates to a method for making a rodent with a humanized IL-7 gene, comprising modifying the germline of the rodent by replacement of rodent IL-7 exons with human IL-7 exons 2 through 6 to form a humanized IL-7 gene, wherein the modification is at an endogenous rodent IL-7 locus and wherein the humanized IL-7 gene is under control of endogenous rodent IL-7 regulatory elements. The present invention also relates to a humanized IL-7 locus comprising a replacement of rodent IL-7 exons at an rodent IL-7 locus with human IL-7 exons to form a humanized IL-7 gene, wherein the humanized IL-7 gene comprises a rodent IL-7 exon 1, a human IL-7 exon 2, a human IL-7 exon 3, a human IL-7 exon 4, a human IL-7 exon 5 and a human IL-7 exon 6, and the humanized IL-7 gene is under control of rodent IL-7 5’ regulatory elements.
