Tumor cells often display increased genomic instability and many anticancer treatments use genotoxic agents targeting DNA. Therefore DNA damage response biomarkers, such as the phosphorylated form of the histone H2AX (?-H2AX), are useful for both tumor characterization and the evaluation of chemotherapy efficacy. However, a major drawback for their use in a clinical setting is their limited accessibility. The molecular chaperone Heat shock protein 90&agr; (Hsp90&agr;), a target of new therapeutic cancer strategies, is often elevated in cancer and has been detected in the serum of patients with various cancer types. Hsp90&agr; represents thus an accessible biomarker for some cancer evaluation. We present here evidences that a phosphorylated form of Hsp90&agr; is a potential surrogate biomarker to follow response to DNA damage induced by treatments in tumours. We recently found that DNA-PK phosphorylates Hsp90&agr; in response to ionizing radiation (P-Thr7-Hsp90&agr;) [1]. Basal levels of P-Thr7-Hsp90&agr; correlated with ?-H2AX levels in a variety of human xenografted tumors in mice. We have previously shown that the treatment with short double-stranded DNA molecules (named Dbait) impairs the repair of irradiation-induced DNA damage in tumors [2,3]. By mimicking DNA double-strand breaks (DSBs), Dbait molecules activate the DNA-dependent protein kinase (DNA-PK), a central DSB signaling enzyme. Dbait treatment induces both nuclear and cytoplasmic phosphorylation of Hsp90&agr;. Moreover, we were able to detect the Thr7 phosphorylated form in the cell culture media of Hsp90&agr;-secreting tumor cells. The P-Thr7-Hsp90&agr; biomarker may therefore represent a surrogate biomarker of genomic instability and DNA damage, allowing both the stratification of cancer patients and monitoring of genotoxic drug responses.