are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and 1,500 species are currently identified.[1][2][3] They are estimated to constitute 1% of all described fungal species.[4] Yeasts are unicellular organisms which evolved from multicellular ancestors,[5] with some species having the ability to develop multicellular characteristics by forming strings of connected budding cells known as pseudohyphae or false hyphae.[6] Yeast sizes vary greatly, depending on species and environment, typically measuring 3–4 µm in diameter, although some yeasts can grow to 40 µm in size.[7] Most yeasts reproduce asexually by mitosis, and many do so by the asymmetric division process known as budding.