Darryl K Shibata University of Southern California (USC) Pathology In theory, it should be possible to reconstruct the past of a human tumor from its genomes. This past can be subdivided into two phases. Between the zygote and the first transformed cell is a single lineage whose genealogy has a phenotype predominately that of a normal stem cell. Mutations that accumulate during this first phase will be “public” and present in all tumor cells. Mutations that arise in the second phase after transformation and during clonal expansion will be “private” and present in only some tumor cells. The subdivision of human tumor mutations into private and public mutations allows inference of the entire tumor ancestral tree from the zygote to present day tumor cells.