What is the clinical value of blood-based biomarkers (plasma phosphorylated tau217 [p-tau217], glial fibrillary acidic protein [GFAP], and neurofilament light chain [NfL]) to detect Alzheimer disease (AD) in clinical syndromes related to frontotemporal lobar degeneration (FTLD)? The gene discussed is GFAP; the disease is early-onset autosomal dominant Alzheimer disease.