Loss of heterozygosity and chromosomal rearrangement of the WWOX gene, which is located at 16q23.3-24.1, have been detected in ovarian, breast, hepatocellular, and prostate carcinomas and in other neoplasias. This gene, which spans the common chromosomal fragile site 16D, contains 9 exons and encodes a 46 kDa WWOX protein that contains 414 amino acids. The evidence from cancer cell lines and primary tumor tissues suggests that WWOX is a tumor suppressor gene and that its inactivation contributes to cancer development. The results from studies of WWOX gene knockout cancer cells and a WWOX knockout mouse model partly confirm this hypothesis. The nature of the various proteins that the WWOX protein can interact with, such as c-Jun, TNF, p53, p73, AP-2 gamma, and E2F-1, suggests that WWOX plays a central role in tumor suppression through transcriptional repression and apoptosis, with its apoptotic function the more prominent of the two. However, there is not universal agreement that WWOX is a tumor suppressor gene. Further analysis is needed to reveal the true nature of WWOX.