Background: Gefitinib binds to the ATP-binding site of EGFR tyrosine kinases to interrupt the abnormally activated anti-apoptotic Ras signaling cascade in malignant cells.Despite its high efficacy against cancers, Gefitnib is also reported to be easily developed acquired resistance by neoplastic cells.The emergence of high-throughput technologies enable the exploration of the underlying drug resistance mechanisms on the level of systems biology.To explore how hepatocellular carcinomas develop resistance against Gefitinib, we obtained the associated genes, analyzed their enrichment distribution on the biological pathways and construct biological association networks.Methods: The cell line HepG2 was cultured by gradually increasing dose of Gefitinib and final dose of 50 lμmoloL-1 in vitro for 12 months to generate its resistance cell line HepG2/gefitinib.The gene expression patterns are given by the CapitalBio mRNA expression microarrays.Genes which are not included in EntrezGene are filtered out.Genes with more than 2-fold expression changes between sensitive and Gefitinib-resistant cells are determined as differentially expressed genes (DEGs).The EGFR-centered network is constructed from the top 6 DEGs.All the enrichment and network analyses are executed by use of the pathway analysis platform Metacore (http://www.genego.com/metacore.php).Results: 379 genes are determined to be DEGs (146 over-expressed and 233 underexpressed).The over-expressed ge nes are mostly distributed in the alternative complement pathways in the immune response, while the under-expressed genes are mostly related to cell development such as the regulation of epithelial-to-mesenchymal (EMT).The constructed EGFR-centered network has 93 nodes (proteins) and 140 edges (interactions).EGFR is the divergence hub of this network.It interacts with the other 35 proteins by effects of binding, phosphorylation, transcription regulation and so on.Conclusions: The presence of redundant and compensatory signaling pathways to EGFR is probably the main reason that HepG2 cells develop resistance to Gefitinib.As is implied in the network analysis, the enhanced activation of integrins can be the reason that Ras cascade is over-activated again when Gefitinib-resistance is developed in liver cancer cells .