Twinning: mechanisms and genetic implications.

In the past, twins have been studied to determine the genetic contribution to various disease processes. Recent work, however, suggests that monozygotic (MZ) twins are not truly identical. Many genetic forms of discordance have been described within MZ twin pairs and may even play a role in causing MZ twinning. Intra-uterine environmental differences in the allocation of the number of cells and in the placental vascular supply to each twin, as well as stochastic development events, may lead to major discordance at birth between the twins of a MZ pair.