This phase 1 study established the recommended dose of eribulin mesylate as 1.4 mg/m2 when administered on days 1 and 8 of a 21-day cycle with appropriate dose adjustment in combination with either weekly trastuzumab (4 mg/kg loading dose, 2 mg/kg/weekly) or tri-weekly trastuzumab (8 mg/kg loading dose, 6 mg/kg/tri-weekly) in Japanese patients with advanced or recurrent HER2+ breast cancer. Here, ERBB2 is linked to breast cancer.