Monozygotic twins concordant for Cayler syndrome.

Deletions within chromosome band 22q11.2 are associated with a variety of conditions, although a simple genotype-phenotype correlation has not been established so far. Environmental factors, chance events, or a second hit theory were supported by two observations of monozygotic twins with 22q11.2 deletions and discordant phenotypes [Goodship et al., J Med Genet 1995;32:746-748; Fryer, J Med Genet 1996;33:173]. We present monozygotic twins concordant for 22q11.2 deletion and Cayler syndrome, favoring the view that there exists a predominant genetic determination of the del 22q11.2 phenotype. As these twins are diamniotic and dichorionic, they may offer a more reliable insight in genetic phenotype determination than the other published, probably monochorionic, twins who may have a discordant malformation by twinning itself.