Background: Hyperactivated epidermal growth factor receptor (EGFR) and/or RAS signaling drives cellular transformation and tumorigenesis in human lung cancers, but agents that block activated EGFR and RAS signaling have not yet been demonstrated to substantially extend patients' lives. The human homolog of Drosophila seven-in-absentia - SIAH-1 and SIAH-2 - are ubiquitin E3 ligases and conserved downstream components of the RAS pathway that are required for mammalian RAS signal transduction. We examined whether inhibiting SIAH-2 function blocks lung cancer growth. Methods: The antiproliferative and antitumorigenic effects of lentiviral expression of anti-SIAH-2 molecules (ie, a dominant-negative protease-deficient mutant of SIAH-2 [SIAH-2PD] and short hairpin RNA [shRNA]-mediated gene knockdown against SIAH-2) were assayed in normal human lung epithelial BEAS-2B cells and in human lung cancer BZR, A549, H727, and UMC11 cells by measuring cell proliferation rates, by assessing MAPK and other activated downstream components of the RAS pathway by immunoblotting, assessing apoptosis by terminal deoxynucleotidyltransferase- mediated UTP end-labeling (TUNEL) assay, quantifying anchorage-independent cell growth in soft agar, and assessing A549 cell-derived tumor growth in athymic nude mice (groups of 10 mice, with two injections of 1 × 106 cells each at the dorsal left and right scapular areas). All statistical tests were two-sided. Results: SIAH-2 deficiency in human lung cancer cell lines reduced MAPK signaling and statistically significantly inhibited cell proliferation compared with those in SIAH-proficient cells (P <. 001) and increased apoptosis (TUNEL-positive A549 cells 3 days after lentivirus infection: SIAH-2PD vs control, 30.1% vs 0.0%, difference = 30.1%, 95% confidence interval [CI] = 23.1% to 37.0%, P <. 001; SIAH-2-shRNA#6 vs control shRNA, 27.9% vs 0.0%, difference = 27.9%, 95% CI = 23.1% to 32.6%, P <. 001). SIAH-2 deficiency also reduced anchorage-independent growth of A549 cells in soft agar (mean number of colonies: SIAH-2PD vs control, 124.7 vs 57.3, difference = 67.3, 95% CI = 49.4 to 85.3, P <. 001; shRNA-SIAH-2#6 vs shRNA control: 27.0 vs 119.7, difference = 92.7, 95% CI = 69.8 to 115.5, P <. 001), and blocked the growth of A549 cell-derived tumors in nude mice (mean tumor volume on day 36 after A549 cell injection: SIAH-2 PD infected vs uninfected, 191.0 vs 558.5 mm3, difference = 367.5 mm3, 95% CI = 237.6 to 497.4 mm3, P <. 001; SIAH-2PD infected vs control infected, 191.0 vs 418.3 mm3, difference = 227.5 mm3, 95% CI = 87.4 to 367.1 mm3, P =. 003; mean resected tumor weight: SIAH-2PD infected vs uninfected, 0.12 vs 0.48 g, difference = 0.36 g, 95% CI = 0.23 to 0.50 g, P <. 001; SIAH-2PD infected vs control infected, 0.12 vs 0.29 g, difference = 0.17 g, 95% CI = 0.04 to 0.31 g, P =. 016). Conclusions: SIAH-2 may be a viable target for novel anti-RAS and anticancer agents aimed at inhibiting EGFR and/or RAS-mediated tumorigenesis.