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In the present study, we addressed the molecular mechanism of the downregulation of reversion-inducing-cysteine-rich protein with Kazal motifs (RECK), a critical tumor suppressor that can potently inhibit angiogenesis and metastasis, in non-small cell lung cancer and its clinical significance. The methylation status of the RECK gene promoter was studied by methylation-specific polymerase chain reaction. RECK mRNA and protein levels were investigated by reverse transcription–polymerase chain reaction and western blot analysis. Downregulation of RECK was observed in 60% of the 55 tumors analyzed. Using methylation-specific polymerase chain reaction analysis methylation of the RECK promoter was detected in 63.6% (35/55) of the tumor tissues. A strong correlation between downregulation and promoter methylation was found in these tumors (P = 0.000005). More importantly, downregulation of RECK significantly correlated with lymph node metastasis (P = 0.038). Mutation of codon 12 of the K-ras gene was detected in 25.5% (14/55) of lung tumor tissues. Statistical analysis indicated that K-ras mutation was linked with RECK promoter methylation (P = 0.047) and downregulation (P = 0.023). Promoter methylation was also detected in human lung cancer cell lines, and the DNA methyltransferase inhibitor 5′-azacytidine reversed the expression of RECK and reduced the invasive ability of these cell lines. Collectively, our results suggest that downregulation of the metastasis suppressor RECK is caused by promoter methylation in non-small cell lung cancer and is associated with K-ras mutation and lymph node metastasis. (Cancer Sci 2007; 98: 169–173)