In the last years histone demethylases were excessively studied regarding their role in cancer development and their involvement in DNA damage response. Especially the members of the Jarid1 demethylase family that are associated with numerous oncogenic diseases came into focus of researchers. Many studies that screened for specific inhibitors were initiated to reveal the exact roles in cell cycle regulation and oncogenic signaling and to target the Jarid1 demethylase family for cancer treatment. In several cancer cell lines I investigated the effects of siRNA-mediated depletion of histone demethylase Jarid1A (KDM5A, RBP2), which demethylates transcription activating tri- and dimethylated lysine 4 at histone H3 (H3K4me3/me2), on cellular proliferation, H3K4 methylation and certain histone acetylation levels as well as on cellular response to radiation. In unirradiated cells Jarid1A depletion leads besides the expected increase in H3K4me3 methylation levels to histone hyperacetylation without affecting cellular growth and proliferation properties. In irradiated cells, depletion of Jarid1A significantly increased cellular radiosensitivity without altering cell cycle regulation. Unexpectedly, the hyperacetylation phenotype did not affect accumulation of the DNA damage response and repair factors γH2AX, 53BP1, BRCA1, or Rad51 at damage sites. It did furthermore not influence the resolution of radiation-induced foci DSB repair pathways as indicated in a DSB reporter assay. Gene expression microarray analysis after Jarid1A depletion and irradiation did not hint at major disturbance of pathways related to DNA damage or general cellular stress response. Single DDR genes that showed a slightly altered expression in the microarray displayed no changes on protein level. I therefore conclude, that the radiation sensitivity observed following depletion of Jarid1A is neither caused by deregulation of typical damage response pathways nor by deficiencies in the repair of DNA double-strand breaks.