Internalization of IMAB027 in various CLDN6+ human ovarian (OC) and testicular cancer (TC) cell lines was assessed by immunofluorescence, flow cytometry, and Fab-ZAP internalization assay....IMAB027–vcMMAE reduced the viability of CLDN6+ OC and TC cells by up to 100% with EC50 values in the ng/mL order....IMAB027–vcMMAE is a specific antibody–drug conjugate against CLDN6 that induces potent antitumor activity in CLND6+ tumor cells in vitro and in vivo.