We initiated the proof‐of‐concept study of ABP‐based single‐molecule protein analysis using prostate‐specific antigen (PSA) as a biomarker of prostate cancer.[18] Currently, antibody‐based detection is commonly used to measure PSA in blood as a biomarker of prostate cancer, and single‐molecule assays such as digital ELISA have also been applied; however, these methods detect the total amount of PSA molecules regardless of their active site accessibility. The gene discussed is KLK3; the disease is Familial prostate cancer.