Three phenotypes of the tumors arising in these patients determine the nature of the TSSC that has a functional initial or truncal Tp53 mutation that initiates tumor formation: (1) A very early age of onset of the tumor formation, (2) the tumor tissue type that is produced at this very early age indicates the TSSC and its clonal expansion, and (3) the excess risk of the tumor tissue type in Li-Fraumeni Syndrome patients compared to the general population. The gene discussed is TP53; the disease is Li-Fraumeni syndrome.