Production of cloned proteins by GMOs

    A gene of interest (for example, insulin or a growth hormone) is engineered into a plasmid containing a tissue-specific promoter. The plasmid is injected into a fertilized sheep ovum and implanted into a foster mother. In a small number (< 1%) of such ova, the plasmid is stably integrated into the host genome: the ovum will develop as a genetically-modified organism (GMO). The famous "Dolly" is such a GMO. When Dolly reaches maturity, the promoter will be activated only in mammary tissue, and the gene of interest will be expressed and secreted into the milk, from which it can be recovered.

    Ideally, the integrated gene will be passed on to Dolly's progeny: in practice, integration is often unstable. She is thus useful as an ordinary breeding ewe: production of new clones every generation is not commercially feasible.