TP53 and Li-Fraumeni syndrome: In this sense, whereas tumor development in HEM mice and Li-Fraumeni syndrome patients, has been associated with a loss of hemizygosity or to a dominant-negative effect of mutated p5323,25–30, around 50% of tumors in HEM mice retain a copy of the WT allele20,23,25, suggesting that reduced dosage of p53 is sufficient for cancer development.