Overall, our study provides a comprehensive assessment of co-occurring KRAS mutations in NSCLC and in vitro evidence of the negative impact of co-occurring KRAS mutations on cellular response to G12C inhibitors, highlighting the need for a comprehensive KRAS tumour genotyping for optimal patient selection for treatment with a KRAS G12C inhibitor. The gene discussed is KRAS; the disease is non-small cell lung carcinoma.