Mammals are diploid organisms whose cells possess twomatched sets of chromosomes, one inherited from the motherand one from the father. Thus, mammals have two copies ofevery gene. Normally both the maternal and paternal copy ofeach gene has the same potential to be active in any cell.Genomic imprinting is an epigenetic mechanism that chang-es this potential because it restricts the expression of a gene toone of the two parental chromosomes. It is a phenomenondisplayed by onlya few hundred of the approximately 25,000genes in our genome, the majority being expressed equallywhen inherited from either parent. Genomic imprinting af-fects both male and female offspring and is therefore a con-sequence of parental inheritance, not of sex. As an example ofwhat is meant by this, an imprinted gene that is active on amaternally inherited chromosome will be active on the ma-ternal chromosome and silent on the paternal chromosome inall males and females.The definition of genomic imprinting is restricted here to“parental-specificgeneexpressionindiploidcells.”Thus,dip-loid cells that contain two parental copies of all genes willexpress only one parental copy of an imprinted gene and si-lence the other parental copy. In contrast, nonimprintedgeneswill be expressed from both parental gene copies in a diploidcell. To understand the concept of genomic imprinting it isimportant to distinguish between imprinted genes and thoseshowing apparent parental-specific expression because ofunequal parental genetic contribution to the embryo. Exam-ples of unequal parental genetic contribution include Y chro-mosome–linked genes present only in males, genes thatescape X inactivation in females (producing a double doseof X-linked gene products compared with males), mitochon-drial genes contributed mainly by the maternal parent, andmessenger RNAs (mRNAs) and proteins present only in thesperm or egg cytoplasm.Manyfeatures of genomic imprinting in mammals make ita fascinating biological problem in postgenomic times. It isintriguing that the subset of genes subject to genomic imprint-ing largely code for factors regulating embryonic and neona-tal growth.Thus,it islikelythat genomicimprintingevolvedtoplay a specific role in mammalian reproduction. It is alsoproviding clues as to a possible evolutionary response to pa-rental conflict, to the adaptation of the maternal parent to aninternal reproduction system, and, perhaps, providing aglimpse of the way the mammalian genome protects itselfagainst invading DNA sequences. Genomic imprinting is anintellectually challenging phenomenon, not least because itraises the question of why a diploid organism would evolve asilencing system that forsakes the advantages of the diploidstate.At this stage of our knowledge, genomic imprinting doesnot appear to be widespread among the four eukaryotic king-doms that include Protista, Fungi, Plants, and Animals. How-ever,itdoesexist,inapossiblyrelatedform,intwoinvertebratearthropods—Coccidae and Sciaridae, and in the endospermof some seed-bearing plants, such as maize andArabidopsis.This distribution indicates that genomic imprinting arose in-dependently at least three times during the evolution of life.Surprisingly, despite this predicted independent evolution ofgenomic imprinting, some similarities among the imprintingmechanism are emerging. It is likely that this reflects conser-vation of basic epigenetic regulatory mechanisms that under-lie both genomic imprinting and normal gene regulation.