The genome of the South American marsupial, Monodelphis domestica, has recently been sequenced. The platypus and tammar wallaby genome projects are also well underway. I will talk about my role in characterizing the immune genes of these species, and will focus primarily on the Major Histocompatibility Complex (MHC). Studying the organization and gene content of the MHC in marsupials has allowed us to predict what the ancestral mammalian MHC looked like and has helped us to understand how our own highly complex immune system evolved