Cancer cells resemble embryonal cells morphologically and share with them characteristics such as reduced differentiation, rapid proliferation rate, and increased motility. Genes expressed in embryogenesis, down-regulated with tissue maturation and reexpressed in cancer, are designated as oncofetal genes, and many of them are used as tumor markers. The H19 gene is an imprinted gene that is expressed from the maternal allele and functions as an RNA molecule. It is abundantly expressed in fetal life and down-regulated postnatally. We have shown oncofetal expression of H19 in human cancer. The study of H19 expression in testicular germ cell tumors of adolescents and young adults, which follow lines of differentiation of the conceptus, demonstrates dissociation between level of expression and monoallelic versus biallelic expression, which are two independent oncofetal characteristics of cancer. Expression of the maternally expressed H19 from the paternal allele in the villous cytotrophoblastic cells of the androgenetic complete hydatidiform mole is designated relaxation of imprinting. H19 is abundantly expressed in the fetal bladder mucosa and in carcinoma of the urinary bladder. It is a marker of early recurrence and may be used as a potential basis for gene therapy.