Epigenetic modification of the genome ensures proper gene activation during development and involves : genome methylation changes ; the assembly of histon and histone variants into nucleosomes and remodeling of other chromatin - associated proteins such as linker histones , polycomb group , nuclear scaffold protein , and transcription factors 这些染色质的再程序化包括dna的甲基化和核小体组蛋白的乙酰化等。组蛋白乙酰化在染色质结构调节、 dna复制、基因转录、细胞分化及癌细胞发育都有研究，但在克隆动物中却是一个有待深入探索的领域。
And the scientists had good reason to be pessimistic : several years of animal cloning work had taught them that most cloned animals never even make it to birth , and the rare ones that do all too frequently have problems ranging from physical deformities to life - threatening medical conditions 科学家们如此悲观是有充分道理的：几年的动物克隆工作使他们明白，大多数克隆动物根本活不到出生的那一天，那些极少数能够顺利出生的动物往往有各种问题从身体畸形到危及生命的种种疾患。
Sheep , mice , cattle , goats and pigs have been cloned , dolly - style , by transferring the nucleus of a cell from an adult animal into a hollowed - out egg . the success rate with cats was as low as with these animals . cc , born two months ago , was the only live birth from 87 cloned embryos that were implanted in surrogate mothers 至今为止，科学家们已经陆续成功克隆了绵羊老鼠牛山羊和猪，这些动物克隆的方式都和第一只克隆动物多利绵羊是相同的，即把成年动物细胞中含有遗传物质的细胞核植入去核的卵细胞中，并使两者融合，融合体分裂发育成克隆动物。
