Xenograft tumors are tumors that are created by transplanting human cells or tissues into animal models, typically mice. These models are used in cancer research to study the growth and behavior of human tumors in a controlled environment. Xenograft tumors can be established using various types of human cancer cells, providing valuable insights into tumor biology, drug efficacy, and potential treatment options. The use of xenograft models allows researchers to test new therapies and better understand the mechanisms underlying tumor growth and progression.