The availability of robust cell-surface markers for identifying and isolating adult stem cells is essential for studying both their normal in-vivo function during tissue renewal and for evaluating their contribution to cancer. We have shown Lgr5, a Wnt target gene expressing a 7-TM receptor that functions as facultative component of the Wnt receptor complex, to selectively mark stem cells in a range of rapidly renewing tissues, including the small intestine, colon, stomach, hair follicle, ovary and developing kidney. Multicolor lineage tracing employing the stem cell-specific Lgr5-CreERT2 line has been used to further dissect how these adult stem cell pools maintain tissue homeostasis and contribute to tissue repair following damage. Targeted in-vivo mutation of the Lgr5+ve adult stem cell pools using the same Lgr5-CreERT2 model has been used to determine the contribution of stem cells to tumor initiation and progression in the gut. A summary of this work will be presented here.