Hope is now high among those affected by Huntington’s too. But until a treatment becomes generally available, their options remain limited. There are medications to help manage some of the symptoms – such as depression, mood swings and involuntary movements – but nothing can yet slow its progression.

Confusion and weight loss always appear as the disease takes hold. The journalist Charlotte Raven has been symptomatic for over seven years. She is quite thin, and keeps up good conversation in a strong voice with no trouble, but she can’t read any more – “the words just don’t go in”. As a writer and editor, she’s acutely aware of what the disease is doing to her, and she misses the bustle of running a magazine; planning and coordinated activity are difficult. “I just sit and listen to Radio 4. Dramas I can’t take in, but news I can do.”

For people like Charlotte and their families, the last great breakthrough was the arrival of testing. A 90–95 per cent accurate test followed the location of the HTT gene in 1983, though it was complicated, needing several family members to make a diagnosis. When the gene was sequenced in 1993, an almost 100 per cent reliable test became available – but for many, this only compounded the misery.

As James says: “When it was discovered, they thought there’d be an influx, but no one’s coming through the door. Thinking about it, why would you want to know?” With no cure available, taking the test means a person has the chance of turning their 50 per cent hope into 100 per cent misery at a stroke. Most decide to wait and see. To this day, only around 20 per cent of the at-risk population in the UK have taken the test.

Elisabeth Rosser, a clinical genetics consultant at Great Ormond Street Hospital in London, has some revealing figures on the psychology of testing. The fully reliable test was offered to 80 people in Oxfordshire who had previously tested positive on the 90–95 per cent test. When she asks me to guess how many opted for it, I plump for 10, thinking I’d better aim low. Only one took up the offer.

“They wanted that 5–10 per cent of doubt and hope,” Rosser explains, the modest chance that their diagnosis was wrong. Conversely, people who had tested negative before were also reluctant to take the new test: “They feared that their near certainty of being disease-free might freakishly be taken away.” But the existence of a treatment would surely change this. Starting treatment before the onset of symptoms is always going to be better.

The main spur to taking the test has been the wish to start a family, and it’s here the broader implications of there being no treatment come into focus. Before testing became available, many of those at risk simply opted not to have children – with a 50 per cent chance that their self-denial was unnecessary. Some gambled. But with the arrival of the test, another option became available: pre-implantation genetic diagnosis (PGD). This, though, brings its own difficulties.

PGD involves in vitro fertilisation, with only embryos free from the Huntington’s mutation being selected. Since 2013 the NHS has offered three free cycles of PGD to at-risk couples. The proviso is that if successful, it won’t fund PGD for a subsequent child. The cost of private PGD – the price of a sibling – is around £15,000.

The test; the wish to have children; PGD; the lack of a treatment – these play out in various permutations in different families. James and his wife wanted healthy children, but PGD normally reveals whether the parent has Huntington’s – and James, like many others in families with a history of the disease, didn’t want to know. There is a way PGD can do this, using a protocol called exclusion testing. It involves seeing which grandparents an embryo has inherited its chromosomes from. In the case of James, any embryos carrying a chromosome 4 from his father were excluded.

This worked for James and his wife, but there were complications, and they lost their baby at 12 weeks. “It affected my wife quite badly,” he says. They waited to try again, but gradually it became clear that this was not going to happen. The issue of James’s at-risk status became a problem, and they split up.

James came to decide that if he were to think about entering a new relationship and having children, it would be better if he had been tested. In September 2017, he received his result: he doesn’t have Huntington’s. He has a clear future.

For Louise* and Simon*, things started similarly but then took a different course. “We didn’t know that HD was in the family until my dad was diagnosed,” Louise recalls. She and her sister were now at risk, and the dilemma of whether or not to get tested left them “going round in circles”. But, she says, “PGD was one area where we felt we could do something positive – move forward.” Like James and his wife, Louise and Simon opted for exclusion testing. This was successful, and they now have a boy.

But Louise knows she would have acted differently if a treatment had been available: “It’s a game-changer. If there was a treatment, I would want to know my status, so I could start ASAP if I was gene-positive.”

The way James and Louise and Simon engaged with the options is by no means typical – awareness of the availability of PGD and uptake of all the possible interventions are low. Nayana Lahiri, a clinical geneticist at St George’s Hospital in south London, reports that prenatal testing has remained at a constant, fairly low level in the UK since the mutant gene was sequenced in 1993. On average, just over 20 tests have been conducted each year. PGD has become more popular – at least 50 cycles were completed in the UK in 2015 – but this represents a recent sharp rise. From its introduction in 2002 until 2009, the annual number of cycles was never more than 20.

What is evident is that, although the disease is implacable, every family deals with it in their own way. Charlotte Raven had a daughter, Anna, in 2005, just months before her father divulged that he’d been diagnosed. She did decide to take the test, which proved positive, and there followed “years of agony”. She “worried about Anna being on her own”, so she and her husband decided to have another child. “I remember thinking should I get the embryos tested,” she says, but in the end, rather than try to “medicalise everything”, she decided to proceed without. As she put it in an article in the Daily Telegraph: “In the event, hope won over apprehension and our son was born, five years after his sister.” Both, though, will have to wait until they are 18 to take their own tests to see whether they have inherited the faulty gene.