In the second part (phase 1b) of the first trial, safety and activity of the RP2D will be tested in three expansion cohorts: a HER2-positive advanced breast cancer cohort, a HER2-low advanced breast cancer cohort, and a HER2 positive gastric cancer cohort (clinicaltrials.gov identifier: NCT02512237). Here, ERBB2 is linked to breast carcinoma.