A biological marker, or biomarker, is a measurable indicator of a biological state or condition. It can be a substance produced by the body, such as proteins, genes, or hormones, that reflects normal or abnormal processes, or a response to a treatment. Biomarkers are often used in medical research and clinical practice to diagnose diseases, monitor health, or predict outcomes. Biomarkers can be found in various biological samples, including blood, urine, or tissue. For example, cholesterol levels can serve as a biomarker for heart disease, while prostate-specific antigen (PSA) is a biomarker used to screen for prostate cancer.