The M1 RNA subunit of Escherichia coli RNase P is a ribozyme responsible for the catalytic activity of the complex. It removes the 5' leader sequence from tRNA precursors to form mature tRNAs. M1 recognizes its target mainly on the basis of its structure and this allows the design of modified ribozymes engineered to destroy other molecules without the need for special sequences in the targeted mRNAs. M1 is thus an ideal tool to eliminate the tumourigenic chimeric messengers created after chromosomal translocations. These results have direct implications for cancer therapeutics and molecular biology in general.