Background: Cetuximab was approved for treating EGFR-expressing metastatic colorectal carcinoma (mCRC) without patient stratification. Subsequent retrospective clinical studies resulted in an exclusion criterion for patients with KRAS mutations at codons 12 and 13. However, only a fraction of patients with wtKRAS benefit from the treatment. Recent analyses indicated that patients with KRAS mutation at codon 13 can also benefit. We set out to investigate whether KRAS mutations, or any other factors, are good biomarkers for CRC-cetuximab therapy patient stratification. Methods: We have established patient derived xenografts (PDX) from treatment-naive Asian CRC patients to discover biomarker predictive of cetuximab response. We conducted a clinical trial class study (“patient avatar trial”) of cetuximab using a randomly selected cohort of 26 EGFR+ PDXs. The antitumor activities were analyzed against KRAS mutation status and a published expression-signature. Results: We identified 12/26 (~46%) as cetuximab responders and 14/26 non-responders (defined by 50% tumor growth inhibition threshold). All 14 non-responders are mutated for one or more of KRAS, BRAF (V600E), EGFR, AKT and PIK3CA oncogenes. In contrast, 5/11 responders analyzed are wild-type for all these genes. Importantly, 5/11 responders have activating KRAS mutations at codons 12/13, contradictory to the currently practiced clinical exclusion criteria, but consistent with more recent clinical findings. Most interestingly, the observed cetuximab activity in this cohort of 26 PDXs has a surprisingly strong correlation to a published RAS pathway score. Conclusions: Our results indicated strong predictive power of cetuximab-CRC-PDX trial and RAS signature, and warrant prospective clinical studies for further confirmation.