Please cite this article in press as: Matias A, et al. Monozygotic twins: Ten reasons to be different. Diagn Prenat. 2014. http://dx.doi.org/10.1016/j.diapre.2013.09.003 ARTICLE IN PRESS DIAPRE-105; No. of Pages 5 d i a g n p r e n a t . 2 0 1 4; x x x(x x) :xxx–xxx Diagnóstico Prenatal www.elsevier.es/diagnprenat Original Article Monozygotic twins: Ten reasons to be different Alexandra Matias a,* , Sara Silva a , Yolanda Martins a , Isaac Blickstein b,c a Department of Obstetrics and Gynecology, University Hospital of S. Joao, Medical Faculty of Porto, Portugal b Department of Obstetrics and Gynecology, Kaplan Medical Center, Rehovot, Israel c The Hadassah-Hebrew University School of Medicine, Jerusalem, Israel a r t i c l e i n f o Article history: Received 28 August 2013 Accepted 12 September 2013 Available online xxx Keywords: Monozygotic twins Genotype Phenotype Epigenetic processes a b s t r a c t MZ twins, though arising primarily from a single zygote and considered genetically iden- tical, will not ever be absolutely the same. Post-fertilization events such as chromosomal mosaicism, skewed X-inactivation and imprinting mechanisms, as well as other epigenetic mechanisms are responsible for the putative differences between MZ twins. Numerous discordant MZ twins have been reported in the literature, including discordance for malfor- mations or genetic diseases, lateral asymmetry, discrepant growth and intrauterine death of the co-twin. This discrepancy for the disorder may be valuable in the analysis of the effect of a disease upon gene expression or on phenotype variation, and have long-term implications. With today’s whole-genome sequencing technologies and increasing evidence for genetic and epigenetic differences in some MZ twins, the understanding of these differences is a whole new field of research. We reviewed the genotypic and phenotypic differences between MZ twins and discuss ten main reasons for being different. © 2013 Asociación Espa ˜ nola de Diagnóstico Prenatal. Published by Elsevier España, S.L. All rights reserved. Gemelos monocigóticos: 10 razones para ser diferentes Palabras clave: Gemelos homocigóticos Genotipo Fenotipo Proceso epigenético r e s u m e n Si bien se considera que los gemelos monocigóticos surgen de un solo cigoto y son genéti- camente idénticos, nunca lo son absolutamente. Ciertos procesos posfecundación, como el mosaicismo cromosómico, los mecanismos de inactivación sesgada del cromosoma X e impronta genética, así como otros mecanismos epigenéticos son responsables de las supuestas diferencias entre los gemelos monocigóticos. En la literatura se han publicado numerosos ejemplos de gemelos monocigóticos «discordantes». Esta discrepancia puede resultar valiosa en el análisis del efecto de una enfermedad sobre la expresión genética o la variación fenotípica, y tener implicaciones a largo plazo. Con las tecnologías de secuen- ciación genómica actuales y el creciente cuerpo de evidencia sobre las diferencias genéticas y epigenéticas en algunos monocigóticos gemelos, la comprensión de estas diferencias se ha convertido en un nuevo campo de investigación. * Corresponding author. E-mail address: matiasalexand@gmail.com (A. Matias). 2173-4127/$ – see front matter © 2013 Asociación Espa ˜ nola de Diagnóstico Prenatal. Published by Elsevier España, S.L. All rights reserved. http://dx.doi.org/10.1016/j.diapre.2013.09.003