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e14670 Background: A major determinant of cancer resistance and recurrence is the presence of quiescent cancer cells. It has long been known that quiescent cancers are resistant to a wide variety of anti-cancer therapeutics relative to actively proliferating cells. However, the presence and relative size of the quiescent compartment is not revealed by standard H&E staining and morphological tumor analysis and, therefore, the data demonstrating its importance and prevalence by tumor type are lacking. DYRK1B kinase (serine/threonine-protein kinase dual-specificity tyrosine-(Y)-phosphorylation regulated kinase 1B) is associated with growth and survival of many types of cancer cells and was shown to play a key role in maintaining the cancer cells in reversible quiescent state by stabilizing the CDK inhibitor p27 and by inducing the breakdown of cyclin D isoforms. Methods: We conducted an immunohistochemical study of 80 human glioblastoma (GBM) tumors (primary / treatment-naive, Grade IV) and 10 cases of normal postmortem brain tissues for expression of DYRK1B, and quiescence and proliferation markers p27 and Ki-67, respectively. For subsection of these tissues, high levels of DYRK1B and p27 expression were observed. Results: In this study, we investigated the functional and therapeutic relevance of DYRK1B expression in glioblastomas, the most common brain cancer in adults. It is an aggressive, highly resistant to treatment tumor type with median survival time of less than 16 months following surgery and treatment. Conclusions: There was a statistically significant correlation between immunostaining of DYRK1B and p27, indicating the presence of dormant cancer cells mixed with the proliferating cancer cells within the patients' tumors. We suggest that DYRK1B inhibitors in combination with chemotherapeutics agents have a potential to improve patient treatment outcome and prolong survival.