evidence: Naturopaths have long been obsessed with a gene called MTHFR. Now vaccine skeptics are testing for it too. [...] The paper was titled "Genetic Basis for Adverse Events after Smallpox Vaccination," and it came up in 2016 when a vaccine-skeptical doctor tried to argue that it explained her patient’s development delays. The court was not persuaded, but Reif’s co-authors began hearing of yet other doctors using DNA tests to exempt patients from vaccines. Just this month, San Francisco’s city attorney subpoenaed a doctor accused of giving illegal medical exemptions from vaccination, based on "two 30-minute visits and a 23andMe DNA test." On anti-vaccine blogs and websites, activists have been sharing step-by-step instructions for ordering 23andMe tests, downloading the raw data, and using a third-party app to analyze a gene called MTHFR. Certain MTHFR mutations, they believe, predispose kids to bad reactions to vaccines, possibly even leading to autism—a fear unsupported by science. [...] 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.)