DNA polymerase theta (Pol θ) is involved in an end-joining pathway of DNA double-strand breaks. Over expression of Pol θ is found in many cancers, including stomach, colon, breast and lung cancers, and correlated with poorer patient survival. Because suppression of gene expression of Pol θ results in sensitivity of cells to ionizing radiation and some DSB-inducing drugs, Pol θ is a validated anti-cancer drug target.