TP53 and neoplasm: p53S18A knock-in mice, in which serine 18 was mutated to a non-phosphorylatable alanine.353 Phosphorylation of p53 serine 18 does not affect the stability of p53 protein, but contributes to the activation of p53 target genes, thus participating in p53-dependent apoptosis and delayed tumor suppression.354 p53S23A knock-in mice, in which serine 23 was mutated to a non-phosphorylatable alanine.