A measurable signal in your blood, tissue, or elsewhere that tells you something is happening biologically — a proxy for what you’re actually trying to track.In peptide research, relevant biomarkers include IGF-1 levels (GH axis activity), inflammatory cytokines (IL-6, TNF-α, CRP), and tissue-specific enzymes. Biomarkers used as primary endpoints in clinical trials are surrogate endpoints — they substitute for harder-to-measure clinical outcomes. A drug that improves a biomarker does not necessarily improve the clinical outcome the biomarker is supposed to represent.