As we discussed in the introduction to this chapter, studying animal cells in culture is much easier than studying cells in intact animals, partly because they can easily be subjected to a variety of manipulations. Cultured cells are particularly useful for the elucidation of fundamental processes. One way to understand a biological process is to interfere with a specific constituent in the cell and assess the outcome—this approach is like trying to understand how a car works by removing components and seeing what goes wrong. In some cases, human diseases associated with genetic defects in specific cell components can be analyzed using cells cultured from the patients. For example, analysis of cultured cells from patients with a genetic defect resulting in hypercholesterolemia—who have elevated blood cholesterol leading to heart disease and stroke—was critical in elucidating the basic steps of receptor-mediated endocytosis (see Chapter 14). In addition to relying on naturally occurring genetic lesions, we can manipulate cultured cells to interfere with expression of specific components. As we will see in Chapter 6, it is possible to decrease the expression of a specific protein in cultured cells by selectively “knocking down” the corresponding mRNA and then assess what effect this change has on particular processes in the cell. Chapter 6 also describes very recently developed techniques that can be used to inactivate specific genes in the genomes of cultured cells and thereby assess how the complete loss of specific RNAs and proteins affects cell functions.