AbstractPurpose and Experimental Design: To identify novel biomarkers and therapeutic targets for lung cancers, we screened for genes that were highly transactivated in lung cancers using a cDNA microarray representing 27,648 genes. DLX5 gene, a member of the human distal-less homeobox transcriptional factor family that is expressed during early embryonic development, was found to be overexpressed in the great majority of lung cancers. Tissue microarray consisting of archival non–small cell lung cancer samples from 369 patients was applied to examine the clinicopathologic significance of DLX5 protein. A role of DLX5 in cancer cell growth and/or survival was investigated through small interfering RNA experiments.Results: Northern blot and immunohistochemical analyses detected expression of DLX5 only in placenta among 23 normal tissues examined. Immunohistochemical analysis showed that positive immunostaining of DLX5 was correlated with tumor size (pT classification; P = 0.0053) and poorer prognosis of non–small cell lung cancer patients (P = 0.0045). It was also shown to be an independent prognostic factor (P = 0.0415). Treatment of lung cancer cells with small interfering RNAs for DLX5 effectively knocked down its expression and suppressed cell growth.Conclusions: These data implied that DLX5 is useful as a target for the development of anticancer drugs and cancer vaccines as well as for a prognostic biomarker in clinic.