Disclosed herein are rodents (such as, but not limited to, mice and rats) genetically modified to comprise a humanized Cxcl13 gene. The rodents disclosed herein have been shown to support better engraftment and proliferation of human cells such as chronic lymphocytic leukemic cells. Compositions and methods for making such genetically modified rodents, as well as methods of using such genetically modified rodents for testing candidate therapeutic agents (e.g., candidate anti-cancer drugs), are provided.