A substance being studied in the treatment of some types of cancer, including Wilms tumor, neuroblastoma, rhabdomyosarcoma, and other types of soft tissue sarcoma. Lorvotuzumab mertansine contains a monoclonal antibody that binds to a protein called CD56 on some types of cancer cells. It also contains an anticancer drug called DM1, which may help kill cancer cells after lorvotuzumab mertansine binds to them. Lorvotuzumab mertansine is a type of antibody-drug conjugate. Also called BB-10901.