AbstractThe process of chromatin diminution inParascarisandAscarisis a developmentally controlled genome rearrangement, which results in quantitative and qualitative differences in DNA content between germ line and somatic cells. Chromatin diminution involves chromosomal breakage, new telomere formation and DNA degradation. The programmed elimination of chromatin in presomatic cells might serve as an alternative way of gene regulation. We put forward a new hypothesis of how an ancient partial genome duplication and chromatin diminution may have served to maintain the genetic balance in somatic cells and simultaneously endowed the germ line cells with a selective advantage.