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A new member of the growing family of metastasis suppressors identified in prostate cancer.

The molecular understanding of cancer metastasis has taken yet another step forward with findings published in this issue of the Journal by Fu et al. (1). The authors report that restoration of Raf kinase inhibitor protein (RKIP) expression is associated with the inhibition of prostate carcinoma metastasis but not with the suppression of tumorigenicity. In addition, the authors report an inverse association between RKIP expression and both the stage of disease and Gleason score. These properties epitomize metastasis suppressor genes, a recently described category of molecules defined by their ability to suppress metastasis without blocking tumorigenicity (2). RKIP is the thirteenth metastasis suppressor described in the literature for which functional data exist (the others are Nm23, KISS1, KAI1, BRMS1, TIMPs, E-cadherin, MKK4, TXNIP, CRSP3, DRG-1, SSeCKs, and RhoGDI2). Several recent reviews summarize what is known regarding the mechanisms of action of metastasis suppressors and the clinical and experimental data supporting their classifi-cation as metastasis suppressors (3–5). Additional molecules whose expression is associated with cancer progression but for which functional data are not yet published have also been described [reviewed in (4) ], suggesting that the number of metastasis suppressors is likely to grow.