Background: RAS status in tumor tissue is a predictive factor for the efficacy of anti-epidermal growth factor receptor (EGFR) antibodies therapy in patients (pts) with metastatic colorectal cancer (mCRC). In mCRC pts with tumors harboring RAS mutation, anti-EGFR antibodies therapy has the lack of clinical benefit; thus, the survival of RAS mutant mCRC pts is shorter compared to RAS wild mCRC pts. Recently, several studies have shown that colorectal cancer tissue had heterogeneity of gene profiling, and the analysis of circulating tumor DNA (ctDNA) in blood samples could detect the change of gene alterations in tumors which were caused by chemotherapy. Bouchahda et al., demonstrated that nearly half of the pts with RAS mutant mCRC had no detectable RAS mutation in ctDNA after first-line chemotherapy; moreover, some of the pts had clinical benefits from post-anti-EGFR antibodies therapy. Therefore, anti-EGFR antibodies therapy might become a novel treatment option for RAS mutant mCRC pts without RAS mutations in ctDNA after chemotherapies. Other report also showed the frequency of no RAS mutations in ctDNA was about 1%. There have been few studies to prospectively evaluate the RAS status in ctDNA for mCRC pts with RAS mutant tumors treated with standard chemotherapies. We, therefore, conducted an observational study to evaluate RAS mutations and the mutation allele frequency in ctDNA for mCRC pts with RAS mutant tumors who were treated with first- or second-line chemotherapy. Methods: This study is a multi-center observational/translational study in 67 facilities. The key eligibility criteria are as follows: 1) Eastern Cooperative Oncology Group Performance status 0-1, 2) histologically proven unresectable mCRC, 3) RAS mutation in tumor tissue, 4) refractory or intolerable after response to prior fluoropyrimidine-containing regimen. OncoBEAM RAS CRC kit is used to investigate RAS status in ctDNA just after first- or second-line treatment in enrolled pts. The primary endpoint is the frequency of pts without RAS mutations in ctDNA. Secondary endpoints include mutation allele frequency of RAS in ctDNA and clinical outcomes of pre- and post-treatments (overall response rate, disease control rate, overall survival, and progression-free survival). As the exploratory analysis, gene alterations related to the resistant mechanism of anti-EGFR antibodies (BRAF, PIK3CA, ERBB2, MET) are analyzed in pts without RAS mutations in ctDNA. We assume that no RAS mutations are observed in ctDNA in at least 1 % pts. We expect that one patient with no RAS mutations in ctDNA is detected in 100 pts; thus, the sample size of 300 pts is set for our study. Accrual is starting in April 2021. Clinical trial information: UMIN000043442.