The gene, known as MCR-1, was first identified in China in November 2015 in Escherichia coli samples from pigs, pork products, and a handful of human cases. It has since been detected in more than 30 countries, including the United States. The emergence of the resistance gene was believed to be connected to widespread use of colistin in Chinese agriculture. China banned use of the drug in animal feed in 2016, based in part on the findings of that study.Though there have been few cases so far of human infections involving the gene—which has mostly been found in animals—MCR-1 has become a significant public health concern because colistin is one of the few antibiotics left that can be used to treat multidrug-resistant infections. And because the gene is carried on mobile pieces of DNA called plasmids, it can be passed not only to different strains within a single family of bacteria—such as E coli—but also to different types of bacteria.Among the most worrisome scenarios is the emergence of bacteria that harbors the MCR-1 gene along with other antibiotic-resistance genes. If the superbug carbapenem-resistant Enterobacteriaceae (CRE) were to acquire the gene, for example, it could present clinicians with infections that are nearly impossible to treat with current antibiotics.