In theory this makes the pig tissue a closer match for human transplant operations.
What is special about the pigs?
The pigs have been genetically engineered in a way that should make their organs more suitable for transplant into humans.
A gene that would normally lead pig organs to be rejected by the human body has been switched off.
In theory this makes the pig tissue a closer match for human transplant operations.
However, the human immune system is extremely complex and further work will be needed to find out if this really is the case.
The fact that the genetically engineered pigs are clones would potentially allow a ready supply of organs to be produced. However, the cloning process must be made more efficient if this is ever to become a reality.
Why pigs?
Many experts regard pigs as the most suitable animals for breeding for organs for a number of reasons:
The pig clones, produced by PPL Therapeutics, and similar research by other biopharmaceutical companies, are the first steps towards providing animal organs and tissues for human transplants (xenotransplantation).
If xenotransplantation becomes a reality, it will be a multi-billion dollar business. This is one reason why the companies involved are keen to report any progress as soon as possible.
However, there are still major problems to be overcome. One obstacle is the theoretical possibility that pig tissue could infect patients with pig viruses.
An investigation by British scientists into the risks of animal transplants into organs has warned that a virus called porcine endogenous retrovirus can infect human cells in the laboratory.
A year ago, the UK Government agency responsible for regulating xenotransplantation research said human trials remained some way off. It also found fault with overzealous researchers for creating what it called "inflated expectations about the promise of xeno-organ transplants".
What is likely to happen next?
The work by PPL Therapeutics has yet to be published in a scientific journal. Many scientists will be waiting to see whether the research can be reproduced.
PPL says one of its first targets is to produce pig pancreatic tissue that can be transplanted into human diabetic patients who lack insulin because of defective cells in the pancreas.
The company says experiments will be carried out on monkeys first.
Human clinical trials are not likely to take place for several years.