Some reports have demonstrated that testing of DNA from a single colorectal tumour tissue block will wrongly assign KRAS wild-type status in 8–11.6% of patients.24,25 Thus, the sole reliance on RAS mutation results obtained from a primary tumour sample might misinform effective treatment of residual systemic disease, imposing significant costs both clinically and financially. The gene discussed is KRAS; the disease is colorectal neoplasm.