XKR4 and attention deficit-hyperactivity disorder: Recently, a gene with preferential expression in cerebellum was found to be nominally associated with ADHD: two family-based genetic association studies confirmed a suggestive association of a single-nucleotide polymorphism (SNP) in the XKR4-gene (XK-Kell blood group complex subunit-related family, member 4) (Lantieri et al., 2010; Neale et al., 2008).