Thus, in the groups of CN, AD, and Down syndrome, less likely affected by non-AD pathology, low Aβ1–42/Aβ1–40 ratio values, but not low Aβ1–42 levels alone, were associated with high concentrations of markers of neurofibrillary pathology and neurodegeneration (pTau181, tTau, and NfL). The gene discussed is NEFL; the disease is Alzheimer disease.