It is estimated that 100 to 200 genes in the human genome are imprinted. These are expressed only from one parental allele, although having the same DNA sequence. Several of the known imprinted genes have been shown to be involved in fetal and placental growth, and imprinting defects can lead to embryonic, developmental defects as well as cancer. This mechanism of gene regulation probably emerged 150 million years ago in mammals. The reasons for the appearance and maintenance of this phenomenon throughout evolution are still controversial.