Antigen binding fragment (BFab) derived from antibody (huDS6) targeting a tumorassociated mucin 1–sialoglycotope antigen (CA6) was developed. We synthesized a companion diagnostic PET tracer by radiolabeling B-Fab  with 64Cu to measure CA6 expression on cancer tissues prior to anti-CA6 huDS6-DM4 drug conjugate (antibody drug conjugate: ADC) therapy for ovarian and breast cancer patients