In at least single eukaryote, Tetrahymena, the pre-rRNA molecule holds an intron. Erasing of the intron in running processing of the pre-rRNA does not require the assistance of any protein! Instead, in the presence of guanosine, GDP, GMP or GTP, the intron excises itself is a phenomenon known as self- splicing. This was the first expression of ribozymes, which is, catalytic RNA molecules which catalyze particular reactions. The Self-splicing introns have also been discovered in some eukaryotic mRNAs and even peptidyl transferase a main activity in protein synthesis is now known to be a ribozyme.