The platypus and the echidna are the only extant examples of monotremes or egg-laying mammals. The platypus is very unusual in appearance, covered in thick brown waterproof fur similar to a mole, with a duck-like beak, beaver tail and otter feet. It weighs between 0.7-2.4kg and has an average length of 50cm. It uses its beak for electro-location of prey as it hunts for worms, larvae and shrimps on the riverbed, mostly at dusk/dawn and night. Its vision is thus, relatively poor. On land it lives in a burrow along the riverbank. Platypuses are unique to Australia although fossil relatives have been found in Argentina. Evolutionary studies suggest that monotremes diverged from the mammal line prior to placentals and marsupials.

The platypus has 10 pairs of sex (XY) chromosomes, and sex determination has significant features in common with the system of ZZ/ZW chromosomes found in birds. The platypus lays 1-3 reptile-like eggs which take around 10 days to hatch after developing in utero for about 28 days. The young suckle milk from grooves on the mother's abdomen as they have no teats. Platypuses represent an evolutionary link between mammals and birds and reptiles.

For the Ensembl build gene models were generated using sets of potential orthologs to genes from other mammalian species as well as chicken. The genome contains 1.9Gb with 21,698 protein-coding genes and 28,002 predicted transcripts.