It has been suggested that the actions of a protein found in modern chicken eggs may make the answer different. In the uterus, chickens produce ovocleidin-17 (OC-17), which causes the formation of the thickened calcium carbonate shell around their eggs. Because OC-17 is expressed by the hen and not the egg, the bird in which the protein first arose, though having hatched from a non-reinforced egg, would then have laid the first egg having such a reinforced shell: the chicken would have preceded this first 'modern' chicken egg. However, the presence of OC-17 or a homolog in other species, such as turkeys, and finches suggests that such eggshell-reinforcing proteins are common to all birds, and thus long predate the first chickens.