It took a long time to produce the first cloned pigs, not least because pig pregnancies do not continue unless there are several piglets developing. In the year 2000 the first five pig clones were produced by the lab which had produced Dolly.

Pigs are particularly interesting because of their potential as a source of organs for human transplants. In 2002, five genetically engineered piglet clones were produced – these had all been modified so that the main antibody on their cells which causes an immune response in people was missing.

Then in 2003 a cloned, genetically modified mini pig was born. Goldie not only had the genes which give rise to the main immune response knocked out, but the pig was also a special tiny size which would mean the organs are more likely to match human ones.

RDS/Wellcome Trust Photographic Library

Putting animal organs in human beings raises the risk of cross species infections, although so far the evidence is promising. Ethically, animals cannot give consent to being modified and cloned to be used as an organ source – they must therefore be regarded as organ sources rather than organ donors (see pushing the boundaries – xenotransplantation).