Somatic DNA mutations accumulate continuously in cells of multicellular organisms, and are passed on to the cells' progeny. Closely related cells tend to share more mutations, and cell lineage relationships can therefore be reconstructed from the degree of mutational similarity. Frumkin and colleagues isolated single cells from several foci of a mouse lymphoma and adjacent normal tissues by laser microdissection, and performed a phylogenetic-like analysis of mutations accumulated in the cells, resulting in a reconstructed cell lineage tree. Analysis of the tree reveals the monoclonal nature of the tumor (shaded sub-tree), its founder cell, its age, the physically coherent manner in which it grew, and the numbers of cell divisions leading to each cell (represented on the left as a vertical scale bar). The tumor is also associated with a specific mutation in p53 that was found in some of its cells (arrows). The approach described here can be useful for investigating basic properties of the growth and spread of early tumors, and the clonal dynamics that underlie cancer progression and metastasis formation. For details, see the article by Frumkin et al. on page 5924 of this issue.