An amino acid which is used by plants and bacteria to make methionine, threonine, and isoleucine (three of the twenty amino acids used to make proteins). Homoserine is similar to the amino acid serine (another of the twenty amino acids used to make proteins), except it has an extra methylene group. It is also formed when cystathionine is metabolized into the amino acid cysteine (another of the twenty amino acids used to make proteins).