The associated SNP lies within a region of SNPs in moderate LD that span a gene desert of about 450 kb in size on 12q21.31 mapping neither to any annotated gene nor to predicted human mRNAs with the exception of some small human expressed sequence tags (EST, fig. 1A and tab. S1). The closest RefSeq annotated gene is SLC6A15 (NM_182767) which ends 287 kb further distal to the region of association. It belongs to the solute carrier 6 (SLC6) gene family and codes for a sodium-dependent branched-chain amino acid transporter (Broer, 2006). SLC6A15 gene expression is highest in the human brain as well as the brain of other vertebrate species (UniGene, 2009). In rodents and humans, SLC6A15 expression has been shown to be restricted to neurons with strong expression in many brain regions including the hippocampus (Anonymous2009; Farmer et al., 2000; Masson et al., 1996). Other genes distal to SLC6A15 are TSPAN19, LRRIQ1 and ALX1 (fig. 1A). Their function is largely unknown and their expression levels are low in the vertebrate brain (Anonymous2009; UniGene, 2009). The nearest gene on the
