Mirroring the numerous monogenic disorders that result from mutations in phosphoinositide phosphatases, GWAS studies have linked mutations in the 5-phosphatase gene SYNJ1 causing PARK20 to Alzheimer’s disease (Herold et al., 2016), while another study has directly correlated SYNJ1 polymorphisms to age of onset in familial Alzheimer’s disease (Miranda et al., 2018). This evidence concerns the gene SYNJ1 and familial Alzheimer disease.