The purpose of this course is to introduce the differences and applications of various gene expression systems, aiming to have students learn how to choose a proper expression system based on the source and property of the target protein to be expressed, as well as the purpose of the expression. The class will firstly introduce gene-expression related knowledge and techniques, then several gene expression systems will be described, including the E. coli system, mammalian expression system, baculovirus-insect cell system, Xenopus oocytes, and cell-free systems.