The present invention relates to an isolated insulator DNA molecule consisting of the 5' and 3' regions of the porcine wrap gene locus and a method for insulating the expression of an introduced heterologous gene from silencing and variegation effect in chromatin into which the gene has integrated. The invention is also directed to mammalian cells comprising said insulator DNA molecule and non human animals obtained therefrom, which are preferably able to express a polypeptide of interest in milk.