evidence: A Facebook post from mid-November 2018 went viral in late August 2020, receiving tens of thousands of interactions on Facebook. The post incorrectly claims that birthmarks are "indicators" of the presence of the methylenetetrahydrofolate reductase (MTHFR) gene, involved in the production of vitamin B9, and that the gene is associated with a higher risk of adverse reactions to vaccines. Because the MTHFR gene is present in every person, the author of the post is likely referring to mutations in the gene, rather than the gene itself. While some rare MTHFR mutations are the cause of a metabolic disorder, most of the mutations in this gene are very common among the population and do not represent a health risk. As we explain below, there is no scientific evidence that mutations in the MTHFR gene are associated with the presence of birthmarks or a higher risk of adverse reactions to vaccines. [...] Also contrary to the claim, there is no scientific evidence that birthmarks, such as stork bites, sugar bugs, and sacral dimples, are indicators of mutations in the MTHFR gene. These skin marks result from the overgrowth of blood vessels or pigment cells, but the cause of such overgrowth is unknown. In general, birthmarks are harmless and fade over time without requiring treatment. However, experts recommend monitoring birthmarks that are large, located along the spine, or interfere with normal activity.