Immunoglobulins recognize foreign antigens and initiate immune responses such as phagocytosis and the complement system. Each immunoglobulin molecule consists of two identical heavy chains and two identical light chains. The immunoglobulin heavy locus, also known as IGH, is a region that contains a gene for the heavy chains of human antibodies (or immunoglobulins). This locus includes V (variable), D (diversity), J (joining), and C (constant) segments. During B-cell development, a recombination event at the DNA level joins a single D segment with a J segment; the fused D-J exon of this partially rearranged D-J region is then joined to a V segment. The rearranged V-D-J region containing a fused V-D-J exon is then transcribed and fused at the RNA level to the IGHM constant region; this transcript encodes a mu-heavy chain. Later in development B cells generate V-D-J-Cmu-Cdelta pre-messenger RNA, which is alternatively spliced to encode either a mu or a delta-heavy chain. Mature B cells in the lymph nodes undergo switch recombination so that the fused V-D-J gene segment is brought in proximity to one of the IGHG, IGHA, or IGHE gene segments, and each cell expresses either the gamma, alpha, or epsilon heavy chain.

This strain is an Ighj-deletion model. In the homozygous BALB/c-Ighj KO mice, the J-segment of the Ig heavy chain locus is completely deleted, resulting in the inability of the cell to produce a recombinant version of the complete heavy chain variable region. The B cells of BALB/c-Ighj KO mice have undergone dramatic changes in the developmental process and cell number, which can be used as an animal model of B cell immune deficiency. BALB/c-Ighj KO mice retain other immune cells except for B cells, so the presence of other immune cells can be detected in BALB/c-Ighj KO mice.