TP53 and neoplasm: The recent understanding that most normal tissues with intact TP53 genes generally are low-dose hypersensitive (LDHS) and low-dose apoptotic (LDA) implies that the well-known universal clinical fractionation window at ≈2 Gy/Fr defines the optimal tolerance level of most organs at risk and not the optimal tumor dose per fraction at least when using intensity-modulated radiation therapy (IMRT).