NTRK2 and bipolar disorder: A specific BDNF SNP (single nucleotide polymorphism) and other BDNF haplotypes, linkage peaks and NTRK2 (TrkB) gene variations are associated with autism, bipolar disorder, schizophrenia, OCD and ADHD (Correia et al. 2010; Nishimura et al. 2007; Fanous et al. 2004; Alonso et al. 2008).