What’s changed? The basics of genetics research. With MTHFR, opponents of vaccines have been able to exploit a lag between the advance in scientific knowledge and widespread understanding of it. In the past 15 years, mainstream genetics research has evolved—but in ways that are not always obvious to the public.

Read: Big Pharma would like your DNA

A lot of that confusion is in fact centered around MTHFR. In the early days of genetics research, scientists looking at a small number of genes in a small number of people found that certain MTHFR variants were linked to a range of maladies: blood clots, cancer, heart disease, pregnancy complications. This seemed to make sense, as MTHFR codes for an important enzyme in the body. But as scientists went from looking at hundreds to thousands to hundreds of thousands of people, they realized many of those variants were extremely common, found in up to 40 percent of the population in some cases.

More importantly, those associations with various diseases just didn’t hold up in bigger data sets and with better statistical tools. They had been flukes all along. MTHFR is not alone in this. The scientific literature is littered with “candidate genes” that turn out not to explain much at all. The American College of Medical Genetics and Genomics currently does not recommend testing for MTHFR.

But the word about MTHFR was already out. And the bold claims about the gene have lingered on alternative and naturopathic medicine sites. “It’s been hard to reverse because if you go to historical literature, you can find some literature to support your claim,” says Elizabeth Varga, a genetics counselor at Nationwide Children’s Hospital. “Without a big picture, without knowing the full story, I think that’s where people who want to exploit the information, they can exploit the information.” And now, mail-in DNA tests have made it relatively easy for anyone to get tested.

In 2017, 23andMe published a blog post noting that MTHFR was the “most asked-about gene by 23andMe customers.” Its scientists concluded that common MTHFR variants were not clinically useful to test. (That’s why anti-vaccine doctors have to download the raw 23andMe data and analyze MTHFR using a third-party tool such as Genetic Genie or Promethease.) The company told me it does not condone the use of its test to grant vaccine exemptions and pointed toward a disclaimer about using its raw data for medical or diagnostic reasons.

Interest in MTHFR and vaccines specifically seemed to have ticked up in 2016, when California outlawed personal and religious exemptions from vaccination. Doctors could still grant medical exemptions, however, and anti-vaccine websites started sharing possible reasons for doing so. MTHFR was listed as one of several, and for evidence they linked to Crowe and Reif’s paper. (Interestingly, the paper actually identified a second gene called IRF1 as well, but the focus has always been on MTHFR, perhaps because the latter gene was already familiar to the naturopathic community.)