Sunakawa Y, et al. - Carcinoembryonic antigen (CEA) response has been reported to be interrelated with clinical outcomes of 1st-line cet-based therapy. Early tumor shrinkage (ETS) is thought of as an on-treatment biomarker for chemotherapy outcomes, so researchers analyzed clinical biomarkers to see if one could predict outcomes earlier. Study participants were 69 patients who could be assessed for CEA at baseline and at 4 weeks, and with observed survival time from two phase 2 trials of 1st-line therapy for KRAS exon2 wild-type mCRC; JACCRO CC-05 of cet plus FOLFOX (UMIN000004197) and CC-06 of cet plus SOX (UMIN000007022). According to findings, a 50% decrease in CEA at 4 weeks is an early on-treatment biomarker for first-line cet-based therapy in mCRC, and it may predict results earlier than ETS. In addition, the CEA response at 4 weeks might differentiate between patients that benefit more from this treatment in left-sided tumors.