Merrimack Pharmaceuticals is conducting a phase II clinical trial of the new drug MM-121. MM-121 is an antibody (a type of immune system protein) that targets a protein called ErbB3 or HER3, which plays a role in tumor growth and survival. ErbB3 is closely related to the protein EGFR. EGFR inhibitors like erlotinib (Tarceva) are used to treat non-small cell lung cancer (NSCLC). The study examines whether combining Tarceva with MM-121 is more effective than Tarceva alone. The trial involves three patients groups: group A (NSCLC patients without mutations in the EGFR gene who have not previously been treated with EGFR inhibitors), group B (patients with EGFR mutations who have not received EGFR inhibitors), and group C (patients who previously received EGFR inhibitors and whose cancer worsened despite the treatment). Group A enrollment has just been completed, group B enrollment is ongoing, and studies of group C were completed recently.