Abstract Comprehensive serach for human cellular response to ionizing radiation (IR) is indispensable strategy to understand the molecular basis under tumor radiotherapy. In a previous study using HiCEP, we found that ASPM (Abnormal spindle-like microcephaly-associated) or the most common-type microcephaly (MCPH5) gene is selectively down-regulated by IR. Here we first demonstrate that down-regulation of ASPM by siRNA synergistically enhanced radiosensitivity in some immortalized human cell lines. Both constant-field gel electrophoresis and gamma-H2AX foci assay exhibited impaired DNA double-strand breaks (DSB) in the cells treated with the siRNA. Greater extent of chromosomal instability was also common in the treated cells than untreated control. Getting together, ASPM (MCPH5) is a novel participant in the DNA double-strand break repair pathways. Although roles of microcephalins (MCPH1, pericentrin, ATR) have recently been proposed on DNA damage repair and check point regulation, this is the first report on ASPM. In clinical aspects, ASPM would be a promising target for the future cancer treatment as well as a useful biomarker in the prognostic strategy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5544.