TP53 and neoplasm: However, while many tumor-suppressor genes are inactivated by deletions, truncating mutations, or epigenetic mechanisms, missense mutations leading to expression of functionally altered full-length mutant proteins with single amino acid substitutions comprise approximately 75% of all p53 alterations, with less than 10% of cancers harboring a truncated form of the protein (21), thus emphasizing the importance of studying the full-length protein.