Heterokaryotypic monozygotic twins discordant for Down syndrome (DS) are very rare, with an incidence estimated to be less than 1 over 7,000,000 pregnancy in the general population. Sharing the same genetic patrimony, except for an additional chromosome 21 for one of them, any gene-expression difference between them could be attributed only to the supernumerary chromosome 21 and not to polymorphic variability in the rest of the genome. The setting up of a prospective longitudinal study will offer the major advantage of allowing genetic and epigenetic comparisons between them and to obtain important information on the impact of the environment in which they live and grow up.