Accession ID of any bidirectional partners as described by Engstrom et al (2006) <i>PLoS Genet</i> 2:e47 (for mouse) and Trinklein et al. (2004) <i>Genome Res</i> 14:62-6 (for human). A bidirectional partner is defined as a transcript that is initiated in the opposite direction to a gene that occurs within 500 bp (for mouse) or 1000 bp (for human).