Marsupials split off from the main mammalian lineage millions of years ago, and have been studied by biologists for years to discover what they can tell us about ancient mammals, particularly the evolution of mammalian sex chromosomes. Plus they are cute! The first marsupial genome has been sequenced and analysed from a tiny opossum named Monodelphis domestica, reported in the current issue of Nature 447, 167-177; 2007. This article, together with an archive of articles on marsupial genetics and genomics, is presented in a feeely available Nature web focus.