This study will evaluate safety and tolerability to estimate the MTDand/or recommended dose.

This is a phase I, open-label, dose-escalation study to establish the MTD and/or RDE of LJM716 as single agent in Japanese patients that have advanced solid tumors. The study consists of a dose escalation part and a dose expansion part.

Inclusion Criteria:

1. Patients with the following indications:

   i) (Dose escalation part only): HER2 overexpressing locally advanced/ metastatic breast cancer or gastric cancer for which no effective treatment option exists:

   • For breast cancer: documented 3+ by immunohistochemistry, or amplification by in situ hybridization
   • For gastric cancer (including GE junction tumors): documented 3+ by immunohistochemistry, or 2+ by immunohistochemistry and amplification by in situ hybridization OR, ii) (Dose escalation part only): Recurrent or metastatic SCCHN regardless of HER2 status for which no effective treatment option exists OR, iii) Recurrent or metastatic ESCC regardless of HER2 status for which no effective treatment option exists

2. ECOG Performance Status of 0-2

3. Must have recovered from the adverse effects of any prior surgery, radiotherapy or other antineoplastic therapy. Alopecia and CTCAE Grade 1 peripheral neuropathy is acceptable.

4. Willingness and ability to comply with all study procedures

5. Written informed consent obtained prior to any screening procedures

6. During dose expansion part of the study, patients must have at least one measurable lesion as defined by RECIST v1.1 criteria