Following assay validation, we used enzyme‐linked immunosorbent assay to measure CSF TMEM119 in 174 patients from six diagnostic groups: Alzheimer's disease (AD, n = 35), amyotrophic lateral sclerosis (ALS, n = 33), cerebral microangiopathy (CM, n = 25), frontotemporal lobar degeneration (FTLD, n = 28), Lewy body diseases (n = 21), and non‐neurodegenerative controls (n = 33). The gene discussed is TMEM119; the disease is early-onset autosomal dominant Alzheimer disease.