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The runt-domain transcription factor Runx3plays crucial roles during development such as regulating gene expression. It has been shown that Runx3is involved in neurogenesis, thymopoiesis and functions like a tumor suppressor. Runx3null mouse die soon after birth as a result of multiple organ defects. Runx3null mouse lung shows an abnormal phenotype and loss of Runx3induced remodeling in the lung. Interestingly, lung adenocarcinoma is observed in Runx3heterozygous mice at 18 months of age. During lung development various cellular and molecular events occur such as cell proliferation, cell death, differentiation and epithelial–mesenchymal transition (EMT). To understand the specific lethal events in Runx3null mice, we examined cellular and molecular networks involved in EMT, and EMT inducers were quantified by RT-qPCR during lung development. Excessive EMT was observed in lungs at PN1 day in Runx3null mice and PN18 months in Runx3heterozygous mice. Pharmacologic inhibition of EMT was used to curb tumor progression. In this study, U0126 was injected to pregnant mouse for inhibition of pERK signaling. After U0126 treatment, life spans of newborn mice were increased and lung hyperplasia was partially rescued by down-regulated cell proliferation and EMT. Our data suggest that Runx3is involved in crucial regulation of alveolar differentiation and tumor suppression in developing mouse lung.