Mutation of the p53 tumor suppressor gene is the most common genetic alteration in human cancer, p73, a member of the p53 family, has been found to exhibit activity similar to that of p53, including the ability to induce growth arrest and apoptosis, p53 and p73 have a high percentage of similarity at several domains, including the DNA binding domain. This domain in p53 is the location of missense mutations in many human cancers. Mutant p53, which cannot suppress cell proliferation, has been found to have a dominant-negative activity that inactivates wild-type p53. To determine the effects of mutant p53 on wild-type p73, we have established cell lines expressing both mutant p53 and wild-type p73 in a dual-inducible system. This system expresses mutant p53 in a tetracycline-repressible system and p73β in an ecdysone-inducible system in a p53-null lung carcinoma parental cell line. We have found that wild-type p73β, in the presence of mutant p53, retains the ability to transactivate p21 and suppresses cell growth through induction of both cell cycle arrest and apoptosis. In addition, in cell lines expressing wild-type p53 and wild-type p73β, we have found that these proteins cooperate to additively transactivate p21 and suppress cell proliferation.