During the last part of the twentieth century, research on human cancer increasingly focused on the molecular basis of this disease. These studies have identified many facets of cellular transformation, including aberrant cell cycle regulation, inhibition of programmed cell death or apoptosis, impaired DNA damage repair systems, genomic instability, and altered signal transduction in an increasing number of pathways. However, we have only begun the formidable task of identifying all the genes responsible for these cellular and genetic alterations. Over the past four decades, investigators have developed multiple models to investigate the processes of malignant transformation, including transformation of cells both in vitro and in the animal by chemical carcinogens or oncogenic viruses. Immortal human and rodent tumor cell lines have proved especially amenable for identification of oncogenes and tumor suppressor genes. In order to map and identify functional tumor suppressor genes directly, we and others have used the technique of microcell hybridization to transfer chromosomes from normal human cells into human tumor cell lines (1 ,2 ). By this approach, we first showed that introduction of a normal human chromosome 11 into a Wilms’ tumor cell line causes a complete suppression of tumorigenicity (3 ). After our initial study, multiple laboratories have established the validity of this experimental system by the demonstration that transfer of specific human chromosomes into a variety of different human tumor cell lines results in the suppression of tumorigenic potential (4 –9 ). In this chapter, we outline a general strategy for mapping the location of tumor suppressor genes by microcell-mediated chromosome transfer (MMCT).