Genomic imprinting is the biological process whereby a gene or genomic domain is biochemically marked with information about its parental origin. Genomic imprints may be covalent (DNA methylation) or non-covalent (DNA-protein and DNA-RNA interactions, genomic localization in nuclear space), and the process of imprinting encompasses the specialized nuclear enzymatic machinery which maintains parental epigenetic markings throughout the cell cycle. For some genes, only one copy is expressed. Expression of these genes is variable depending on which parent the gene came from, a process known as genomic imprinting. In this way, the active gene is preferentially transmitted from one parent over the other.