A patient with NSCLC and an EGFR exon 19 deletion from the original biopsy had cfDNA collected after becoming refractory to erlotinib; cfDNA revealed, in addition to an EGFR exon 19 deletion (6.42%), EGFR T790M (0.65%) and PIK3CA E545K (0.67%) mutations not previously identified in the earlier tissue testing, which can credibly explain the emergence of resistance. The gene discussed is EGFR; the disease is non-small cell lung carcinoma.