Scientists have moved a step closer to making animal-to-human organ transplants a reality by developing a new pig whose organs won't be rejected.

Scottish-based PPL Therapeutics Plc the firm behind Dolly, the world's first cloned sheep has produced five piglets lacking the rejection-causing gene.

Humans have a powerful or hyperacute rejection response when our antibodies attach to sugar molecules on the surface of the transplanted pig cells.

When the molecules attach, human antibodies kill the pig cells. In the genetically modified line of miniature swine, the gene producing the sugar has been knocked out or eliminated so the rejection process can't happen.

Pig kidneys, hearts and other organs could help solve a dire shortage of donated human organs. Pigs are considered the best candidates to use because their organs are about the same size as those in adult humans.

But some scientists worry the pigs could spread porcine viruses to transplant patients. Others question the ethics of using raising animals for their organs. PPL hopes to start clinical trials on humans within four years.

The five newborns piglets, named Joy, Angel, Mary, Star and Noel, huddled in the corner of their pen of artificial turf at PPL's news conference on Wednesday. PPL has not yet published the results in a scientific journal.

Rival biotechnology companies are also working on creating genetically modified pigs for transplant purposes.

On Thursday, the U.S.-based Immerge BioTherapeutics Inc. reported their line of modified pigs did not spread a porcine retrovirus to human cells in a tests tube study. The study was published online by the journal Science.