Transplantation of skin or whole organs (kidney, hear, lung, pancreas) has been attempted in humans with varying success. Transplantation across major hisoincompatibility barriers, as between different species, is associated with rapid graft rejection by the recipient. When transplantation is accomplished between subjects matched closely for histocompatibility, graft rejection occurs more slowly. The mechanisms of homograft rejection are complex and not yet completely understood, but they probably involve destruction of the grafted cells by recipient cells sensitized against surface histoincompatibility antigens present on grafted cell membranes. In addition, in some instances a graft vs. host reaction may involve damage to the recipient's cells by the graft cells. Genetic engineering offers the hope of cures for many inherited diseases, once the problem of low efficiencies of effective transfer of genetic material is overcome. Another development has been the refinement of the technique called cloning which produces large numbers of genetically identical individuals by transplanting whole cell nuclei. With other techniques, scientists can isolate sections of DNA representing single genes, determine their nucleotide sequences, and reproduce them in the laboratory. This offers the possibility of creating entirely new genes with commercially or medically desirable properties.