This is the reason why the same mutation in the inherited form, when it acts as an initial or truncal mutation in a tissue specific stem cell, causes a different tumor than when it arises as a spontaneous mutation in a later order of selected mutations (retinoblastomas, Tp53 mutations, PTEN mutations, BRCA-1, 2 mutations, etc.). The gene discussed is TP53; the disease is retinoblastoma.