Scientists have continued working on cloning mammals other than sheep in the four years since Dolly was successfully cloned. Those mammals in which they have achieved success include cows, pigs, and mice.

A group of Texas researchers reported on their progress in cloning cattle in the journal Theriogenology. The scientists managed to clone 13 calves from a male Holstein steer cell line. They succeeded in implanting 13 fetuses in host cows. Eight were born alive, four were stillborn, and one was spontaneously aborted (miscarried). Five of the eight live calves were apparently healthy at birth, although one died at six weeks of age. The other three calves born alive all suffered from neonatal respiratory distress, one of which died. This calf was found at autopsy to have had elevated blood pressure in the circulation to the lungs, and this finding was also found in pathological examinations of the stillborn calves. Six of the surrogate mother cows also suffered complications during their pregnancies with these clones. Thus, while cattle ranchers might look longingly to cloning as a method of producing herds of high-producing milk cows or prime beef cattle, the logistics of cloning cattle demonstrate that the technology has years of refinement ahead before it can be used successfully and cost effectively.

Scientists from the University of Tennessee have cloned a calf they have named Emma. Emma was cloned from a cow that was prone to the disease mastitis, an udder infection that reduce milk production. They next hope to clone calves from cows that share the genetic tendency to be resistant to this disease. Researchers at the University of Vermont are also experimenting with cloning techniques to produce calves that would also be resistant to mastitis, using DNA replacement techniques.

With the passage of time, the publication of results like those above, and more experience, researchers are finding themselves marginally more successful at cloning animals. Scientists at the biotechnology company Infigen reported in the journal Nature Biotechnology on their success at cloning pigs. They were able to produce four healthy cloned piglets in two litters. The piglets were produced by somatic nuclear transfer and in vitro fertilization techniques. The researchers expressed their optimism that these techniques could be combined with genetic treatments. Pigs are potentially good sources of organs and tissues for human use, and genetic modification might reduce the likelihood that those organs would induce immune responses and rejection by the human recipients. Successful pig cloning could go beyond industry interests in meat production and have real benefit in the arena of organ and tissue transplantation.

Scientists have also been successful at cloning mice, albeit at very low rates. Researchers from the University of Hawaii School of Medicine published a report in Molecular Reproduction and Development that outlined their work on cloning mice. They used eggs from adults of various sorts, including hybrids and inbred mice, and they used different cells as sources of the nuclei inserted into the eggs. They had at best a 2% success rate in producing offspring. They were able to conclude that adult cells were no better or worse than fetal cells as sources of the genetic material for the clones, but that success would be limited until the reasons for the low rates were elucidated.