In classical genetics, the DNA’s sequence was thought to be the only determinant for heritability. More recently, research has shown that also non-genetic information can be inherited. The field dedicated to this area is called epigenetics (), which literally means “on the gene”. In particular, molecular marks, like methyl groups, have shown to be heritable. Their presence or absence in DNA control regions controlling gene expression (so called promoters) can influence if a gene becomes active, and if so at what level. It is well know that some chemicals can influence the performance of methylating enzymes. Such influences can be particularly relevant if they happen during the first few days of pregnancy, when the fertilized egg is just starting to develop into an embryo. This stage is marked by a complete removal of epigenetic methylation marks, and a subsequent resetting of these marks. A disruption of this process can lead to permanent health issues in the adult, many years after this molecular event has occurred. Not all compounds can exert this influence, and not all regions on the DNA are affected in the same way, meaning that such effects are likely specific for individual compounds. Furthermore, other epigenetic mechanisms are known, for example affecting the DNA-surrounding chromatin layer. Recent studies also show that chemically induced epigenetic alterations can be inherited over 4 generations, that so  even though in the past they have not been considered permanent like DNA mutations (). There is controversy surrounding findings of epigenetically inherited effects caused by chemical exposures during prenatal development. More research is needed to reproduce first findings.