‘I was looking for information on how these gene-mods are tested,’ Charlie said. ‘I can see that someone would be willing to try an experimental treatment that might cure terminal cancer, but there are hundreds of gene-therapy mods. Who tests the long-term effects and how can you know they’re safe?’

‘I’m very conservative,’ Mango said. ‘There are tens of thousands of gene-therapy templates that you can download and synthesise. I only do mods that people I trust in the online modding community have already tried, and I make it clear to my patients that even the best mods carry a one-in-four-hundred risk of autoimmune disease, or tumours.’

‘So have any of your patients ever got sick?’ Charlie asked, uncomfortable with the idea that a mistake might kill someone.

‘All my patients get a sixty-day cocktail of autoimmune drugs. Mild side-effects are common after gene therapy and I know of a few that have got sick while the modified cells took over, but my only really bad result was an anti-obesity mod.

‘It was designed to affect the hormones that create hunger. It was tested successfully by a Danish university. Unfortunately, all the test subjects were Caucasian and it turned out that it triggers cancer in some racial groups.

‘I did it on a woman born in Peru and within weeks she started passing blood. She got rushed to hospital. The surgeon who cut her open said it was like she had thousands of tiny white teeth growing inside her digestive tract. There was nothing they could do.

‘I felt awful and considered quitting the business. The family weren’t wealthy and we offered to refund my fee to pay for the funeral. But the husband and daughter were very understanding. The woman was desperate to lose weight and my associate in California spoke with them before the treatment and carefully explained that gene therapy carries a chance of unexpected results.’

‘I didn’t realise a mod can affect different ethnicities differently,’ Charlie said.

‘When they first mapped the human genome back in the nineties, scientists thought genes were like rows of switches. A switch for blue eyes, or being short, or being autistic. There are a few genes that work like that, but most influence more than one thing.

‘For instance, twenty genes have been identified that influence how tall a person could be. Then there’s gene expression – which means different genes get switched on and off in the womb and early childhood. That’s why some identical twins look different, though their DNA matches perfectly. Then there’s the junk DNA every human carries. They named it that because it didn’t seem to do anything, like corrupted files on an old hard drive. But it turns out it does influence certain things.’

Charlie’s science-geek brain was ticking happily. ‘So editing genes is easy, but you can’t be sure what it will do, and the same treatment might do different things to different people?’