There is also evidence of a genetic component of the evolution of toothless whales. Multiple mutations have been identified in genes related to the production of enamel in modern baleen whales. These are primarily insertion/deletion mutations that result in premature stop codons. It is hypothesized that these mutations occurred in cetaceans already possessing preliminary baleen structures, leading to the pseudogenization of a "genetic toolkit" for enamel production. Recent research has also indicated that the development of baleen and the loss of enamel-capped teeth both occurred once, and both occurred on the mysticete stem branch.