the e4 allele of the apolipoprotein E gene has been identified as a major genetic risk factor for Alzheimer's. There are three versions (alleles) of the APOE gene; the most common is e3, present in over half the population. Those who inherit one copy of the e4 allele are at higher risk of developing type 2 Alzheimer's, a late-onset form; those who inherit two copies are at greater risk. Most people with familial hypercholesterolemia have 2 copies of the e4 allele. One study suggests having the e4 allele is particularly risky in combination with a small head size. Similarly, calorie and fat intake appear to increase the risk of developing Alzheimer's in those with the allele. An Australian study has more recently identified the -491A allele as another risk factor. The same study found that people with these alleles were more likely to complain of memory difficulties. APOE is located on chromosome 19. A gene on chromosome 10 has also recently been identified as significantly increasing the risk of Alzheimer's when found in combination with APOE e4.