the production of ubiquitin ligases, proteins that regulate muscle protein breakdown. However, Akt is not responsible for all of the observed muscle hypertrophic effects, which are mediated by myostatin inhibition. Thus myostatin acts in two ways: by inhibiting Akt-induced protein synthesis and stimulating ubiquitin-regulated protein degradation. Biological advantage Many different mammalian and avian species produce myostatin, indicating that the ability to produce myostatin was positively selected for. According to some researchers, this is most likely because muscle building and maintenance are biologically expensive processes that require relatively large amounts of energy and nutrients that may not always be abundant, or that could be more useful somewhere else, such as the brain. Having excessive muscle would also negatively affect speed and stamina, as it would require more energy to move at fast speeds for longer periods of time. Effects in animals Mutations Mutations in myostatin do more than just affect the amount of muscle mass an organism can produce; they also have variable effects on other phenotypes for different species.For example, a Belgian Blue bovine with a mutation that inhibits myostatin production will exhibit a dramatic increase in muscle mass but will also lead to dystocia. Other species with myostatin deficiency mutation such as humans or Whippet dogs do not get obstructed labor. Double-muscled cattle After the discovery of the gene encoding myostatin 1997, several laboratories cloned and established the nucleotide sequence of a myostatin gene in two breeds of cattle, Belgian Blue and Piedmontese. They found mutations in the myostatin gene (various mutations in each breed) which in one way or another lead to absence of functional myostatin. Unlike mice with a damaged myostatin gene, in these cattle breeds, muscle cells multiply rather than enlarge. People describe these cattle breeds as "double-muscled", but the total increase in all muscles is no more than 40%. Animals lacking myostatin or animals treated with substances such as follistatin that block the binding of myostatin to its receptor have significantly larger muscles. Thus, reduction of myostatin could potentially benefit the livestock industry, with even a 20 percent reduction in myostatin levels potentially having a large effect on the development of muscles. However, the animal breeds developed as homozygous for myostatin deficiency have reproduction issues due to their unusually heavy and bulky offspring, and require special care and a more expensive diet to achieve a superior yield. This negatively affects economics of myostatin-deficient breeds to