TP53 and neoplasm: Thus, selective accumulation of p53ΔE5-6 protein provides a sensitive marker to identify proliferating GCPs that trigger radiation-induced p53-mediated apoptosis, which otherwise would be eliminated and undetectable in the presence of p53WT function in vivo.34,35 Using the p53ΔE5-6 as a marker, we showed that whereas bulk tumor cells exhibited robust expression of mutant p53∆E5-6 protein, Sox2+ cells rarely expressed a detectable level of p53∆E5-6 protein in SHH-MBs even following high-dose radiation (Figure 5E).