Genomic imprinting is a new concept proposed to explain unusual observations in early mammalian development, the occurrence of certain genetic diseases, genetic anticipation or incomplete penetrance, and tumorigenesis. The basic mechanism of the imprinting has remained obscure, although DNA-methylation, chromatin structure, and/or DNA replication may have a role. Genomic imprinting is a biological phenomenon determined by an evolutionally acquired, underlying system that may control harmonious development and growth in mammals. It is also relevant to the occurrence of some genetic disorders in man.