If there is enough tissue from the original biopsy of a patient’s tumour, this tissue can be tested. If not, a second biopsy to get enough tissue to test may be needed. Some doctors may also want a second biopsy to see if a patient has developed a new mutation called T790M mutation at the failure of first line EGFR-targeted therapy.25 The T790M mutation will cause resistance to EGFR-targeted drugs.25 See About EGFR T790M Mutation section for more information.