Approximately 5% of patients have a family history of ALS [1]. All other cases are considered to have a sporadic form of the disease. ALS is considered to be a disease of complex etiology with both genetic and environmental factors contributing to disease susceptibility [2]. These genetic factors are the subject of extensive research [3]. Multiple genome-wide association studies (GWAS) and candidate gene studies have been carried out, implicating several genes in the susceptibility to ALS [4]–[8], but attempts to replicate most of these genes have proven difficult [9]–[13]. Recently, our group has published a GWAS comprising over 4,800 patients and nearly 15,000 controls and identifying UNC13A and 9p21.2 as susceptibility loci for sporadic ALS [7]. The 9p21.2 locus was recently replicated in an independent set of British patients and controls [12] and also shown to be strongly associated with ALS in Finland [14]. This locus was previously found to be one of the linked loci in families with ALS and frontotemporal dementia (FTD), and it was recently shown that a hexanucleotide repeat expansion in C9orf72 was the basis of this linkage signal [15], [16].
