1. A method for selectively expressing a toxin within a cell comprising administering to the cell a DNA sequence comprising a promoter operatively linked to a transcription sequence; wherein the transcription sequence, when transcribed, produces a messenger RNA sequence that comprises a translatable sequence encoding a toxin, and an untranslated sequence; wherein the untranslated sequence inhibits translation of the toxin sequence under conditions that exist within normal mammalian cells that do not overexpress eukaryotic initiation factor eIF4E; wherein the untranslated sequence allows translation of the toxin sequence under conditions that exist within mammalian cells that overexpress eukaryotic initiation factor eIF4E relative to normal cells, and wherein the untranslated sequence further comprises a hairpin secondary structure conformation having a stability measured as folded state free energy of ΔG<about −50 Kcal/Mol.