PARP1 and neoplasm: DNA-PKcs activation is critical for development of tumor therapy resistance [34, 35], cleaved PARP1 is a marker for apoptosis [36], we isolated tumors from mice, western blot assay showed that MCM3 knockdown inhibited the phosphorylation of DNA-PKcs, and increased PARP1 cleavage (Fig. 5c), suggesting MCM3 knockdown reduced radioresistance.