The present invention provides natural or transgenic galactose deficient cells, tissues, organs and animals that have been genetically modified to compensate for the abnormalities in galactose metabolic pathways. The present invention modifies sugar metabolic pathways to to prevent the deleterious accumulation of sugar metabolites in animals, tissues, organs, cells and cell lines that possess natural or transgenic abnormalities in the sugar metabolic pathways. Such cells, tissues, organs and animals can be used in research and medical therapy, including xenotransplantation.