Lys20p and Lys21p are two isozymes of yeast homocitrate synthase. Their systematic names YDL182W and YDL131W respectively. Lys20p is of molecular size 47. 1 kDa and Lys21p is 48. 6 kDaYeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding. Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation of a string of connected budding cells known as pseudohyphae, or false hyphae, as seen in most molds. Yeast size can vary greatly depending on the species, typically measuring 3–4 µm in diameter, although some yeasts can reach over 40 µm.