LBA444 Background: Patients with KRAS exon 2 codon 12/13 wild-type (wt) mCRC benefit significantly from the addition of cetuximab to first-line FOLFOX4 in relation to response and progression-free survival. Patients with KRAS exon 2 codon 12/13 mutations show no benefit with a trend to worse clinical outcomes. Methods: Tumors from OPUS study patients previously defined as KRAS codon 12/13 wt (n=179) were screened for specific mutations in KRAS exons 3 and 4 (8) and NRAS exons 2, 3, and 4 (18) using BEAMing technology. Treatment outcomes were assessed according to mutation status. Results: RAS tumor mutation status was evaluable for 118/179 (66%) patients. Mutations at the screened loci were detected in 36 (31%) patients. In the RAS wt population, there was benefit associated with the addition of cetuximab to FOLFOX4 (Table). In the overall RAS-mutant population, there was less favorable clinical outcome and no benefit from the addition of cetuximab to FOLFOX4. Conclusions: Patients with mCRC harboring any activating mutation of KRAS or NRAS are unlikely to benefit from the addition of cetuximab to FOLFOX4. Definitive conclusions for patients with new tumor mutations cannot be drawn due to low patient numbers. Restricting cetuximab administration to patients with tumors wt at all such loci might enable the further tailoring of therapy to maximize patient benefit. [Table: see text]