Because of his young age, hereditary variants of CAA and Alzheimer's disease (AD) were excluded (genes tested: APP, NOTCH3, TREX1, HTRA1, ABCD1, AUH, CBS, CLCN2, COL4A1, COL4A2, CSF1R, CST3, CYP27A1, CSTA, DARS2, GBE1, GFAP, GLA, GSN, ITM2B, LMNB1, MMACHC, TREM2, TTR, TYMP, TYROBP). The gene discussed is MMACHC; the disease is early-onset autosomal dominant Alzheimer disease.