ARTICLE ABSTRACTPurpose: Despite continuous improvement of treatment regimes, the mortality rates for non–small cell lung cancer (NSCLC) and head and neck squamous cell carcinoma (HNSCC) remain disappointingly high and novel anticancer agents are urgently awaited.Experimental Design: We combined the data from genome-wide siRNA screens on tumor cell lethality in a lung and a head and neck cancer cell line.Results: We identified 71 target genes that seem essential for the survival of both cancer types. We identified a cluster of 20 genes that play an important role during G2–M phase transition, underlining the importance of this cell-cycle checkpoint for tumor cell survival. Five genes from this cluster (CKAP5, KPNB1, RAN, TPX2, and KIF11) were evaluated in more detail and have been shown to be essential for tumor cell survival in both tumor types, but most particularly in HNSCC. Phenotypes that were observed following siRNA-mediated knockdown of KIF11 (kinesin family member 11) were reproduced by inhibition of KIF11 using the small-molecule inhibitor ispinesib (SB-715992). We showed that ispinesib induces a G2 arrest, causes aberrant chromosome segregation, and induces cell death in HNSCC in vitro, whereas primary keratinocytes are less sensitive. Furthermore, growth of HNSCC cells engrafted in immunodeficient mice was significantly inhibited after ispinesib treatment.Conclusion: This study identified a wide array of druggable genes for both lung and head and neck cancer. In particular, multiple genes involved in the G2–M checkpoint were shown to be essential for tumor cell survival, indicating their potential as anticancer targets. Clin Cancer Res; 19(8); 1994–2003. ©2013 AACR.