Genetically modified non- human animals comprising a human or humanized interleukin- 7 (IL- 7) gene. Cells , embryos, and non -human animals comprising a human or humanized IL -7 gene. Rodents that express human or humanized IL- 7 protein. Genetically modified mice that comprise a human or humanized IL- 7- encoding gene in their germline, wherein the human or humanized IL- 7 encoding gene is under control of endogenous mouse IL- 7 regulatory sequences.