Mysteries of Monozygosity: Theories and Breakthroughs/Twin Research: Rare Case of Lost Twins; Developing a National Twin Registry; Twins' Language and Gesture Delays; DNA Testing for Vanishing Twins/Media Reports: Identical Twins Discordant for COVID Vaccination; World's Oldest Identical Twins; Olympic Athlete Stand-in; Fraternal Twin Football Players

The processes that give rise to monozygotic (MZ) twins remain elusive. This article describes various theories of MZ twinning that have been examined over the years, although they continue to be speculative. It has also been impossible to know if a singleton began life as an MZ twin; however, a critical technological breakthrough can now reveal this important birth history information with a high degree of certainty. The section that follows presents reviews of current research regarding rare twin loss, development of a twin registry, twins' communicative delays and DNA testing for vanishing twins. The article concludes with a survey of newsworthy twins, namely identical twins discordant for COVID vaccination, the world's oldest identical twins, an Olympic athletic stand-in and fraternal twin football players.