Given the extensive history of cancer, the history of patient-derived xenograft (PDX) models is also diffcult to fully recapitulate. In particular, this task is complicated by the irregularity with which PDX models were designated. The current use of “PDX cancer models” is a relatively recent addition to the lexicon. However, the general concept of PDX models—i.e., the transplantation of human cancers into animal models—can be found throughout the chronicles of cancer research. However, it wasn’t until the discovery of host immunity and its crucial role in graft survival that the idea of serial transplantation could be realized. Therefore, in order to better reflect the nature of its history, the term “human tumor models” will be used in place of “PDX models,” except when appropriate. Finally, it is important to also mention the existence of the other labels that have been used, including, but not limited to, “human tumor xenografts,” “xenopatients,” heterotransplant tumor models,” “heterotransplanted human tumors,” and “transplantable tumor models.”