The C9orf72 genetic variant is a “repeat expansion” where a specific six-letter DNA sequence is repeated abnormally, altering the instructions for constructing the C9orf72 protein. As a result, it creates a dysfunctional protein that disrupts waste disposal, produces toxic genetic material, and creates small, harmful proteins that progressively damage the brain. Scientists think these factors might explain how this gene could lead to FTD or ALS.