Yin J, Cohen R, Jin Z, et al. - Researchers sought to determine the influence of primary tumor sidedness (PTS) on results of metastatic colorectal cancer (mCRC) patients. They used the ARCAD database to pool PTS data of 9,277 mCRC patients from 12 first-line randomized trials. Better overall survival and progression-free survival were experienced by left-sided metastatic colorectal cancer patients vs right-sided metastatic colorectal cancer patients. A statistically significant interaction was observed between PTS and KRAS mutation: better prognosis in relation to left-sidedness was evident in KRAS wild-type (WT), but not in KRAS mutated tumors. Findings showed that PTS had a prognostic value limited to the KRAS-WT population. Anti-EGFR efficacy was predicted by PTS, with a statistically significant improvement of survival for left-sidedness mCRC cases. These data indicate that in mCRC, both PTS and KRAS status should be taken into account to decide the treatment approach.