SMPD1 and attention deficit-hyperactivity disorder: Following this lead, SNPs within the target genes (GALC, CERS6, SMPD1, SMPDL3B, CERS2, FADS3, ELOVL5, and CERK) that were found to be associated with ADHD in our study were extracted from the PGC data set, as described in the “Methods” section (a total of 2012 SNPs).