Investigators studying the origin of life have produced ribozymes in the laboratory that can catalyze their own synthesis under certain conditions, such as an RNA polymerase ribozyme. Improved variants of the "Round-18" polymerase ribozyme have been developed. "B6.61" is able to add up to 20 nucleotides to a primer template in 24 hours, until it decomposes by cleavage of its phosphodiester bonds. The "tC19Z" ribozyme can add up to 95 nucleotides with great accuracy.