Good cells gone bad: the cellular origins of cancer.

One issue in human cancer research that remains largely unresolved is the identity of the normal cell type in which tumorigenesis begins. Knowledge of the starting cell populations targeted by transforming events is crucial for characterizing early neoplastic changes and understanding how normal cellular processes become disrupted during cancer development. Recent focus on cancer stem cells (CSCs) has been accompanied by the frequent assumption that these cells derive from normal stem cells. However, evidence from mouse tumor models has demonstrated that cancer can also arise from committed progenitors. This review highlights recent insights gained from studies of the normal cell populations targeted for malignant transformation - in other words, the cellular origins of cancer.