- Part 2, Cohort 2: Histologically confirmed local laboratory testing (immunohistochemistry 3+ staining and/or fluorescence in situ hybridization ratio >= 2.0) HER2+ metastatic breast cancer subjects who are resistant to trastuzumab-containing treatment and progressed on trastuzumab, pertuzumab, a taxane, and/or trastuzumab emtansine. There is no limit regarding the number of prior lines of therapy. Subjects may be enrolled regardless of whether or not their tumor harbors a PAM pathway alteration (documentation of PAM pathway alteration for this cohort is not required).