 This study is a randomized, open-label, international, multi-center, phase 2 study in patients with Heregulin-positive NSCLC that have progressed following no more than three systemic therapies for locally advanced or metastatic disease, of which one must have been an anti-PD-1 or anti-PD-L1 therapy. All patients will initially be screened for heregulin status. Eligible patients will be randomized to receive MM-121 in combination with investigators choice of either docetaxel or pemetrexed versus docetaxel or pemetrexed alone. 