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Cetuximab improves activity of chemotherapy in metastatic colorectal cancer (mCRC). Gene copy number (GCN) of epidermal growth factor receptor (EGFR) has been suggested to be a predictive factor of response to cetuximab in patients (pts) with mCRC; on the contrary, K-ras mutation has been associated with cetuximab resistance.We have conducted a phase II study with cetuximab administered weekly for 3 weeks as single agent and then with 5-fluorouracil and radiation therapy as neo-adjuvant treatment for locally advanced rectal cancer (LARC). EGFR immunohistochemistry expression, EGFR GCN and K-ras mutation were evaluated on diagnostic tumor biopsy. Dworak's tumor regression grade (TRG) was evaluated on surgical specimens.Forty pts have been treated; 39 pts are assessable. TRG 3 and 4 were achieved in nine (23.1%) and three pts (7.7%) respectively; TRG 3–4 rate was 55% and 5.3% in case of high and low GCN, respectively (P 0.0016). Pts with K-ras mutated tumors had lower rate of high TRG: 11% versus 36.7% (P 0.12). In pts with wild-type K-ras, TRG 3–4 rate was 58.8% versus 7.7% in case of high or low GCN, respectively (P 0.0012).In pts with LARC, EGFR GCN is predictive of high TRG to cetuximab plus 5-FU radiotherapy. Moreover, our data suggest that a wild-type K-ras associated with a high EGFR GCN can predict sensitivity to cetuximab-based treatment.