DNA:
NCBI Reference Sequence: NC_000006.11;
370316 bp DNA;
BACH2 consists of 9 exons and 8 intervening introns.
RNA:
NCBI Reference Sequence: NM_021813.2;
9215 bp mRNA.

Transcription produces at least four alternative spliced variants.

By FISH, Sasaki et al. (2000) mapped the BACH2 gene to chromosome 6q15. Southern blot analysis determined that BACH2 is a single-copy gene (Sasaki et al., 2000).

Reference sequence for BACH2 protein: NP_068585.1.
BACH2 contains 841 amino acid residues with a molecular weight of 92536 Da.
BACH2 belongs to the bZIP family and CNC subfamily.
BACH2 contains a BTB domain for protein interaction in the N-terminal region starting at amino acid residue 37 to 103. In the C-terminus from amino acid residue 651 to amino acid residue 666 a basic DNA binding domain, followed by a hydrophobic leucine zipper domain ranging from amino acid residue 674 to 695 responsible for dimerisation.
The four alternative spliced variants generate four different proteins:
1) BACH2-003, transcript ID: ENST00000257749 and protein ID: ENSP00000257749. This transcript contains all 9 exons and have the full length transcript at 9215 bps and a full length protein with 841 aa.
2) BACH2-005, transcript ID: ENST00000343122 and protein ID: ENSP00000345642. This transcript contains 7 exons (1, 4, 5, 6, 7, 8, 9) resulting in a transcript with 3981 bps and a protein with 841 aa.
3) BACH2-002, transcript ID: ENST00000406998 and protein ID: ENSP00000384145. This transcript contains only 4 alternative exons (1, 4, 5, 6) producing a transcript at 780 bps and a protein with 81 aa. This alternative protein only contains the BTB domain.
4) BACH2-007, transcript ID : ENST00000453877 and protein ID: ENSP00000397668. This transcript contains only 5 alternative exons (1, 2, 4, 5, 6), producing a transcript at 868 bps and 81 aa. This alterniative protein only contains the BTB domain.
Liu et al. have, in mice, identified an alternative promoter and new isoforms of BACH2 by using murine insertional mutagenesis (Liu et al., 2009).

Related proteins:
BACH1 and BACH2 are highly homologous proteins with significant similarity to each other in the bZIP and BTB domains. But the expression pattern and function of the two proteins are different. BACH1 is more ubiquitously expressed whereas BACH2 expression is restricted to mononuclear and neuronal cells. (Shim et al., 2006; Hoshino and Igarashi, 2002; Muto et al., 1998; Oyake et al., 1996).

BACH2 is expressed mainly in B-lymphoid cells and in the fetal brain (Muto et al., 1998; Hoshino and Igarashi, 2002). During B cell differentiation BACH2 is expressed from the pro-B cell to the mature B-cell stages but is absent in the plasma cell stage (Igarashi et al., 2007). Its expression is also silenced in primary plasma cells (our unpublished observation). BACH2 is expressed in umbilical cord blood CD4+ T cells (Lesniewski et al., 2008).

BACH2 is predominantly cytoplasmic and migrates to the nucleus upon oxidative stress where it functions as a proapoptotic factor (Muto et al., 2002; Hoshino et al., 2000).

BACH2 protein is a transcriptional repressor or activator. BACH2 forms heterodimers with small Maf oncoproteins (MafF, MafG, MafK) through the BTB domain. It binds to Maf recognition elements (MARE) through the DNA binding domain bZIP. It plays important roles in coordinating transcription activation and repression by MAFK (by similarity) (Oyake et al., 1996).

BACH2 in B-cell differentation:
BACH2 is critical for antibody responses including class switch recombination (CSR) and somatic hypermutation (SH) of immunoglobulin genes (Muto et al., 2004). BACH2 and Bcl-6 cooporates to repress Prdm1, and thereby regulating B-cell differentiation through the germinal center (Ochiai et al., 2008).

Neuronal differentiation:
BACH2 seems to be involed in neuronal differentiation, by upregulating both cytoplasmic and nuclear p21 protein levels. P21 is a cdk inhibitor known to arrest the cell cycle. In N1E-115 cells regulation of differentiation involves a down regulation of cellular proliferation (Shim et al., 2006).

Immune response:
BACH2 is involved in the antiviral response triggered by dsRNA or dsDNA molecular patterns (Hong et al., 2008).

HomoloGene (NCBI) BACH2 conserved in Euteleostomi. Genes identified as putative homologs:
NP_068585.1 BACH2 Homo sapiens
XP_539044.2 BACH2 Canis lupus familiaris
XP_618496.2 BACH2 Bos taurus
XP_232858.3 RGD1562865 Rattus norvegicus
XP_419833.2 BACH2 Gallus gallus
XP_682933.2 bach2 Danio rerio

There are 11 SNPs in coding regions of human BACH2 of which 4 are encoding missense protein residues (NCBI dbSNP).
Bach2 is considered a B-cell specific tumor suppressor since loss of heterozygosity (LOH) is frequently observed for non-hodgkin lymphomas (Sasaki et al., 2000).
High throughput screens using retroviral and transposon insertion into mouse models have identified BACH2 as a common insertion site (CIS) (http://rtcgd.abcc.ncifcrf.gov/). BACH2 expression is in these models disrupted or activated in the resulting B cell tumours.
Homozygous null mice display impaired B cell differentiation and reduced B cell numbers.