The present invention provides peptides corresponding to all or a portion of amino acid residues 12-26 of human p53 protein, which peptides are lethal to malignant or transformed cells when fused to a membrane-penetrating leader sequence. The subject peptides are thus useful in treating neoplastic disease in an animal, preferably a human. Also provided are pharmaceutical compositions comprising the subject peptides admixed with a pharmaceutical acceptable carrier. Methods of treating neoplastic disease in a patient by administering a subject peptide fused at its carboxy terminal end to a membrane penetrating leader sequence are also provided. The present invention also provides replication incompetent Adenovirus (AdV) vectors comprising a promoter sequence operably linked to a nucleotide sequence encoding a subject peptide. Methods of selectively killing cancer cells in a subject by administering a therapeutically effective amount of a subject AdV vector are also provided by the present invention.