Hypermethylation of sex-determining region Y-box 10 (SOX10) has been reported in schizophrenia, whereas in bipolar disorder hypomethylation of HLA complex group 9 (HCG9), ST6 (alpha-N-acetyl-neuranminyl-2,3-beta-galactosyl-1,3)-N-acetylgalactosaminide alpha-2,6-sialyltransferase (ST6GALNAC), and hypermethylation of the serotonin transporter SLCA4 and proline rich membrane anchor 1 (PRIMA1) has been observed (Kato and Iwamoto, 2014). Here, HCG9 is linked to bipolar disorder.