TMPRSS2 and posterior cortical atrophy: Through a combination of lineage tracing experiments coupled with cell-type-specific activation of ERG in basal versus luminal cells, we find that ERG-driven PCa initiate in a rare subset of basal cells present in healthy mouse and human prostates (which we call BasalLum cells) that coexpress various canonical luminal lineage genes including, importantly, TMPRSS2.