The C9ORF72 mutation is called a "dominant" mutation, which means that their children have a 50% chance of inheriting the gene. Most people who inherit the C9ORF72 mutation will develop either ALS or the related disease called fronto-temporal dementia. However, it may be possible for someone to test positive for the C9ORF72 gene mutation and never develop symptoms. Furthermore, in addition to C9ORF72, there are many other gene mutations that can cause ALS. This study will not test these other genes, and therefore a negative test result for the C9ORF72 mutation will not exclude the possibility that you have a heritable form of ALS.