Protein expression in mammalian cells refers to the process of producing proteins using mammalian cell lines. Mammalian cells, such as Chinese hamster ovary (CHO) cells or human embryonic kidney (HEK) cells, are commonly used for this purpose due to their ability to correctly fold and modify proteins. This technique allows for the production of recombinant proteins, which can be used for various applications in research, drug development, and biotechnology. Reference: mammalian transient protein expression service