While YKL-40 shows potential value for early identification along the AD continuum, its limited ability to differentiate between MCI and AD highlights the need for its combined use with other biomarkers in disease staging and progression assessment.<h4>Systematic review registration</h4>https://www.crd.york.ac.uk/PROSPERO/view/CRD420251031837. The gene discussed is CHI3L1; the disease is Alzheimer disease.