DOES NOGGIN CAUSE TWINNING?
Human monozygotic twins account for 1 in 250 live births and are considered genetically identical. The origin of MZ twins is attributed to two or more daughter cells of a single zygote undergoing independent mitotic divisions, leading to independent development and births. To date, the cause of monozygotic twins remains unknown. Human beings and armadillos are the only mammals where monozygosity occurs. We have previously reported two unrelated sets of monozygotic twins from spontaneous dizygotic triplet pregnancies, with a clinical phenotype of facio audio symphalagism (FAS). The dizygotic sibling of each triplet set has a normal phenotype. Spontaneuosly conceived triplet pregnancies occur in 1 in 6500 births in the Republic of Ireland. Noggin mutations occur in approximately 1 in 10000 births. The probability of FAS recurring in monozygotic twins from a triplet pregnancy is therefore highly unlikely. A nonsense mutation in the Noggin (NOG) gene located on chromosome 17 q22 has been i