The gene/chromosome transfer into cultured cancer cells has not only confirmed that genes isolated by the positional cloning method can actually suppress various transformed phenotypes of the cells, but also allowed to speculate cellular functions of cloned or uncloned tumor suppressor genes. One of the important functions of uncloned ones is to induce the cellular senescence program in immortal cancer cells, as revealed by the chromosome transfer. This method has also identified novel chromosomes (or chromosomal regions) carrying putative tumor suppressor genes, which have never been suggested by other approaches, e.g., LOH analyses. Thus, the studies on tumor suppressor genes using cultured cancer cells have different advantages from those of the molecular genetics/molecular biology studies.