Mechanisms for differences in monozygous twins

Monozygous (MZ) twins are often described as being physically and genetical ly identical. Clinical determination of zygosity relies on the assumption t hat any physical differences between a pair of twins imply they are dizygou s. Most twin research relies on the assumption that dizygous twins share ap proximately 50% of the same genes, whereas monozygous twins share 100%. The re is, however, increasing evidence to challenge both these assumptions. In this review, we describe a number of intrauterine effects and genetic mech anisms that may result in phenotypic, genotypic, and epigenetic differences between monozygous twins. Newer molecular techniques are resulting in such differences being increasingly commonly recognised. The potential for diff erences in monozygotic twin pairs is an important consideration for both cl inicians and researchers involved in twin work. (C) 2001 Elsevier Science I reland Ltd. All rights reserved.