The invention belongs to the field of biological medicines, relates to a novel virulence gene for the Alzheimer disease and a medical application thereof, and in particular relates to a mutant of a novel virulence gene of the familial Alzheimer disease, namely a DHHC21 gene. More in particular, the amino acid sequence of the mutant is as shown in SEQ ID NO: 1 or SEQ ID NO: 3. Mutation of S-palmitoyl transferase 21 or a coded nucleotide sequence thereof can induce occurrence of the Alzheimer disease. The novel virulence gene has the potential of preparing a drug for treating and/or preventing and/or diagnosing the Alzheimer disease.