Prior to screening, subjects will have tested positive for a sensitizing EGFR mutation to determine eligibility for treatment with erlotinib. During screening, subject serum samples will be tested using the investigational companion diagnostic (BDX004) test. Only those subjects who have a BDX004 Positive Label will be enrolled. Subject randomization will be stratified by EGFR mutation type and smoking status (ever versus never smokers). Subjects will be designated as never smokers if they have smoked less than 100 cigarettes in their lifetime. Radiographic tumor assessment, to include CT or MRI of chest and abdomen, will be performed every 4 weeks for the first 8 cycles, and every 8 weeks thereafter, using the same imaging modality per subject. Safety assessments will be performed on an ongoing basis.