Microbiology. A genus of ascomycetous fungi of the family Saccharomycetaceae, known as baker's or brewer's yeast. They are normally diploid unicellular fungi that reproduce asexually by budding. This species of yeast is an important model organism for genetics and molecular biology. The entire genome has been sequenced and it is used to do basic research on the cellular mechanism of replication, recombination, cell division, and metabolism.