What type of mutation is alzheimer's disease ?

i need to know if it is a gene mutation or a chromosomal mutation, and what type.
i have not found anything searching the internet, so please help!

Both- read on From first link ***Alzheimers is caused by mutations in three genes. The first mutation to be identified is in the amyloid precursor protein (APP) gene located on chromosome 21. APP is cleaved to form amyloid, a major component in amyloid plaques. Chromosome 21 was viewed as a likely site for Alzheimer's disease-causing mutations for two reasons: (1) it is the location of the APP gene and (2) the pathologic signs of Alzheimer's disease develop at a young age in people with Down syndrome (trisomy 21). The discovery of mutations in the APP gene was viewed by many scientists as the smoking gun indicating that the amyloid gene is the cause of neurodegeneration in Alzheimer's disease. However, mutations in APP are extremely rare and affect fewer than 24 people worldwide.

The presenilin 1 gene (located on chromosome 12) is the most common site of mutations responsible for early-onset Alzheimer's disease. The presenilin gene has a sequence consistent with a membrane-spanning protein, but its function is unknown. The deletion of the presenilin 1 gene in transgenic mice results in early death due to skeletal muscle deformation, impaired neurogenesis, and neuronal cell death. Mutations in presenilin 1 also alter the metabolism of APP. A second presenilin gene (located on chromosome 1) is also implicated in early-onset Alzheimer's disease. Mutations in the presenilin 2 gene are rare and restricted to families that originated in the Volga region of Germany. **

I forget

PSEN1 - Presenilin 1

The presenilin 1 gene (PSEN1) was identified by Sherrington (1995) and multiple mutations have been identified. Mutations in this gene cause familial Alzheimer's type 3 with certainty and usually under 50 years old. This protein has been identified as part of the enzymatic complex that cleaves amyloid beta peptide from APP (see below).

The gene contains 14 exons, and the coding portion is estimated at 60 kb, as reported by Rogaev (1997) and Del-Favero (1999). The protein the gene codes for (PS1) is an integral membrane protein. As stated by Ikeuchi (2002) it cleaves the protein Notch1 so is thought by Koizumi (2001) to have a role in somitogenesis in the embryo. It also has an action on an amyloid precursor protein, which gives its probable role in the pathogenesis of FAD. Homologs of PS1 have been found in plants, invertebrates and other vertebrates.