A team of researchers at MIT revealed in 2007 that they deciphered the DNA of the gray short-tailed opossum, the first marsupial to have its genome mapped. Some scientists believed that opossums—and marsupials in general—have a primitive immune system because they lack key proteins found in placental mammals (the non-marsupials) that regulate immunity. But when the researchers compared the genomes of opossums and humans, they found a surprising number of similar immune-related genes, meaning it’s useful for just the opposite of the expected reason: The gray short-tailed opossum is a nice model for immunology research.