During multistage carcinogenesis, the functions of several key genes involved in cell growth control must be damaged. Such genes include not only those involved in cell cycle control of individual cells, but also those involved in the coordination of cell growth throughout a given tissue through cell-cell communication. The most intimate form of intercellular communication is mediated by gap junctions. Gap junctional intercellular communication (GJIC) is known to transfer small water soluble molecules, including cAMP and IP3, from the cytoplasm of one cell to that of its neighbors; the growth of a given GJIC-associated cell is thus kept in check by other GJIC-connected cells. Most tumor cells have a reduced ability to communicate among themselves and/or with surrounding normal cells, confirming the importance of intact GJIC in growth control. When connexin (gap junction protein) genes are transfected into such cells, normal cell growth control is often recovered. Certain dominant-negative mutant connexin genes can reverse such tumor suppression. While these results suggest that connexin genes form a family of tumor suppressor genes, so far we have found no connexin gene mutations in human tumors; only two connexin gene mutations were found in chemically induced rat tumors. On the other hand, our recent studies suggest that connexin genes may be inactivated by hypermethylation of their promoter regions, suggesting that epigenetic inactivation of connexin genes may be a mechanism of GJIC disturbance in certain tumors. However, in many tumor cells connexins are normally expressed but aberrantly localized. The mechanisms of aberrant localization of connexins include lack of an appropriate cell-cell recognition apparatus and aberrant phosphorylation of connexins. These results suggest that GJIC disorders may occur not only because of aberrant expression of connexin genes themselves, but also as a result of disruption of various control mechanisms of the protein functions.