TMPRSS2 and neoplasm: Through a series of lineage tracing and primary prostate tissue transplantation experiments, we find that tumor initiating activity resides in a subpopulation of basal cells that co-express the luminal genes <i>Tmprss2</i> and <i>Nkx3.1</i> (called Basal<sup>Lum</sup>) but not in the larger population of classical <i>Krt8</i>+ luminal cells.