Two individuals that develop from a single fertilized egg cell by its division into two genetically identical parts. Each part eventually gives rise to a separate individual and these twins share identical DNA sequences. However, even identical twins can differ in patterns of methylation and acetylation of their DNA and in the histone proteins that package the DNA, which result in differences in gene expression (see gene imprinting). Such epigenetic phenomena may explain observed differences in looks, personality, etc. Compare fraternal twins.