Abstract Background and aims: Esophageal squamous cell carcinoma (ESCC) has a high incidence in China, with five-year survival rates in Hong Kong of only 10.7%. Loss of chromosome 13q regions is a frequent event in ESCC. A candidate tumor suppressor gene (TSG), PHF11, located at 13q14.2 was identified from our previous chromosome transfer study and shows 100% down-regulation in 18 cancer cell lines, including 16 ESCC cell lines, compared to expression in an immortalized normal esophageal epithelial cell line. This study aims to investigate the functional role of PHF11 in ESCC and to investigate the possible role of PHF11 in transcriptional regulation or histone modification as the proteins in the plant homeodomain finger (PHF) protein family are reported to contribute to the two implicated functions. Methods: Real-time PCR was carried out to validate the expression of PHF11 in tumor tissues of ESCC patients compared to their respective normal counterparts. The two isoforms of PHF11, PHF11a and PHF11b, were cloned into expression vectors and transfected into ESCC cell lines, KYSE150 and KYSE180, respectively. Clones expressing PHF11 were established for colony formation assay and MTT assay. To identify the subcellular localization of PHF11, subcellular fractionation was performed using immortalized normal esophageal epithelial cell lines followed by SDS-PAGE and Western blotting. Results: Real-time PCR verified that 42% of the 41 patient samples showed at least 2-fold down-regulation in PHF11 expression. The colony formation assay showed that the two isoforms of PHF11, PHF11a and PHF11b, have the ability to suppress cell growth using two ESCC cell lines, KYSE150 and KYSE180. The MTT assay also showed that both isoforms of PHF11 have the ability to suppress cell proliferation. Our data showed that PHF11 is localized in the nucleus of the immortalized normal esophageal epithelial cell lines. Conclusions: It was observed that almost half of the patient samples screened showed a significant down-regulation of PHF11, suggesting its possible role as a candidate TSG. Suppression of colony growth and cell proliferation in the PHF11 expressing clones indicate that the protein may suppress tumorigenicity of ESCC cells. The nuclear localization of PHF11 observed in the immortalized normal esophageal epithelial cell line is consistent with a possible role in transcriptional regulation or histone modification, though further studies are required to further investigate this. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2181. doi:1538-7445.AM2012-2181