2 Background: Efficacy analyses of the randomized phase III CRYSTAL trial have shown a significant improvement in progression-free survival (PFS), overall response, and curative surgery rate when adding cetuximab to FOLFIRI in the first-line treatment of mCRC. Furthermore, KRAS mutation status has recently been shown to relate to outcome in mCRC patients (pts) treated with cetuximab as a single agent or in combination with irinotecan. Efficacy analyses have been repeated to evaluate the influence of KRAS mutation status in first-line pts treated with FOLFIRI with or without cetuximab under controlled study conditions. Methods: Blocks from archived tumor material were available from 587 of 1198 of the total pt population. Isolation of genomic DNA was performed directly from slides (3×10 μm). Determination was done by qPCR-based KRAS mutation analysis of codons 12/13. The KRAS-evaluable pts (n=540) were analyzed statistically to evaluate treatment effect stratified by KRAS mutation status (wild-type [wt] or...