The CCR5 gene has been researched by scientists since the 1990s, and has a number of roles which have not yet properly been uncovered. Loss of the gene’s function is known, however, to increase the risk of potentially fatal reactions to some diseases, and has shown an ability to enhance learning in mice.

Scientists who have analysed He’s slides since he first unveiled his project have suggested that he seems to have produced three different mutations in the twin girls. These mutations are likely to have disabled the gene, with both copies of the gene disabled in one of the twins while the other has at least one working copy.

CCR5 has been proven to protect the body from West Nile virus, transmitted by mosquitoes and to which 20% of those infected develop serious complications such as meningitis or encephalitis. Philip Murphy, an immunologist at the National Institute of Allergy and Infectious Diseases in Maryland who has experimented with the CCR5 function, warned that the twin with only one copy of the gene should receive protection against those effects should she contract the virus, although the other twin may have a higher risk of complications if infected.

Influenza could also pose a danger to the girls. Animal tests have shown that CCR5 helps to recruit key immune cells to fight the virus in the lungs. Without the gene, this defence system fails. Scientists have also found that, among people with multiple sclerosis, those with the CCR5 HIV-resistance mutation deletion are doubly likely to die early than are people without the mutation.

Concerns have been raised over the information in the consent form, however, with none of the potential side-effects having been seemingly communicated to the childrens’ parents, or any other individuals who took part in the research.