Cloning occurs in nature when an early embryo divides into two separate embryos. These monozygotic twins will be genetically identical. This process can be artificially replicated with similar results. Artificial 'embryo splitting' has been used on humans since 1993 when Jerry Hall and Robert Stillman produced genetically identical embryos by splitting early two- to eight-cell embryos into single embryo cells and then placing them in nutrient solution, when they begin to divide again (Kolberg, 1993). Hall and Stillman produced, on average, three cloned embryos from each original. This technique can be used to provide a 'twin' embryo for biopsy, permitting an embryo undamaged by invasive procedures to be available for implantation following the result of the biopsy of the twin, or to increase the number of embryos available for implantation in the treatment of infertility.