A hallmark of cancer is tumor cell heterogeneity. We discovered that most human colorectal cancers (CRCs) are composed of mixtures of phenotypically distinct tumor cells organized into a stem cell hierarchy that displays a striking resemblance to the healthy colonic epithelium. This project intends to answer fundamental questions about tumour cell heterogeneity and specifically about colorectal cancer stem cells (CRC-SCs) including: Are CRC-SCs the only tumour cell population with long term regenerating potential? Will tumour cell plasticity contribute to the regeneration of the CRC-SC pool after therapy? How do common genetic alterations in CRC influence the CRC hierarchy? Do they affect the stem cell phenotype? To answer these questions we have established a collection of patient derived tumours in vitro in the form of 3D structures called organoids and develop genetic strategies using CRISPR/Cas9 technology to track the behavior of CRC-SCs in human cancer. We anticipate that results from this project may help improved therapeutic treatment of CRC patients.