Most of the microorganisms whose genomes have been completely sequenced are problematic pathogens, such as "Haemophilus influenzae", which has resulted in a pronounced bias in their phylogenetic distribution compared to the breadth of microbial diversity. Of the other sequenced species, most were chosen because they were well-studied model organisms or promised to become good models. Yeast ("Saccharomyces cerevisiae") has long been an important model organism for the eukaryotic cell, while the fruit fly "Drosophila melanogaster" has been a very important tool (notably in early pre-molecular genetics). The worm "Caenorhabditis elegans" is an often used simple model for multicellular organisms. The zebrafish "Brachydanio rerio" is used for many developmental studies on the molecular level, and the plant "Arabidopsis thaliana" is a model organism for flowering plants. The Japanese pufferfish ("Takifugu rubripes") and the spotted green pufferfish ("Tetraodon nigroviridis") are interesting because of their small and compact genomes, which contain very little noncoding DNA compared to most species. The mammals dog ("Canis familiaris"), brown rat ("Rattus norvegicus"), mouse ("Mus musculus"), and chimpanzee ("Pan troglodytes") are all important model animals in medical research.