In order to study the functions of a cell's endogenous mutant p53, the p53 protein levels must be knocked-down. Transient transfection of small interfering RNAs is one way to accomplish this. Another is the stable expression of short hairpin RNAs. This chapter presents a method by which a short hairpin RNA (shRNA) targeting p53 is inserted into the genome of a cell via lentivirus infection. These p53 knock-down cell lines are stable and may be grown long term for use in a wide range of applications.