APOE and Alzheimer disease: Costa-Laparra et al. studied the impact of the Apoe ε4 allele co-occurring with a presenilin 1 (PSEN1) gene mutation by analyzing skin fibroblasts from AD patients harboring this combination compared to fibroblasts from persons with 2 ApoE ε 4 alleles and healthy ApoE3/3 controls [175].