Monotremes (egg-laying mammals) are the only extant mammalian outgroup to therians (marsupial and eutherian animals) and provide key insights into mammalian evolution. An international team of scientists has sequenced and analyzed the genomes of the platypus (Ornithorhynchus anatinus) and the short-beaked echidna (Tachyglossus aculeatus), the two extant monotreme lineages, and