Herein, we investigate gene-gene interaction patterns in monogenic AD and FTD, with a focus on genetic variability in key AD (<i>APP</i>, <i>PSEN1</i>, <i>PSEN2</i>) and FTD (<i>MAPT</i>, <i>GRN</i>, <i>C9orf72</i>)-associated genes and their association with cumulative disease incidence. Here, C9orf72 is linked to Alzheimer disease.