**2. IG gene organization and gene rearrangements**

#### **2.1 Gene segments and generation of functional IG genes**

In humans, the immunoglobulin heavy (IGH) locus is present at chromosome 14q32.33, and the IG light lambda (IGL) and kappa (IGK) loci are located at 22q11.2 and 2p11.2, respectively [1]. The immunoglobulin heavy chain variable region (IGH V) gene encodes VH framework regions 1-3 (FR1-3), VH complementarity determining region 1 (CDR1), and VH CDR2, while IGH joining (IGH J) gene encodes VH FR4. VH CDR3 is positioned at the IGH V-IGH D-IGH J junction [2]. The constant sequences of heavy (H) chain are arranged in the following order: μ → δ → γ3 → γ1 → α1 → γ2 → γ4 → ε → α2 in humans (**Figure 1**), or μ → δ → γ3 → γ1 → γ2b → γ2a → ε → α in mice.

#### *Normal and Malignant B-cell*

Similar to T-cell receptors (TCRs), functional genes of immunoglobulins (Igs) are the result of somatic recombination of DNA containing the relatively limited germinal genetic information, using the so-called V(D)J recombination process that occurs between individual genes (also referred to as gene segments) of the variable domains of the H and L chains (or α, β, γ, and δ chains of TCRs). Each of these genes is present in multiple copies in most antigen receptor loci. The locus of IGH genes (like TCR β and TCR δ loci) contains variable (V), diversity (D), and joining (J) genes, whereas the IGL and IGK loci (like TCR α and TCR γ loci) contain only V and J genes [3]. So, individual V, D, and J genes at the IGH locus, and V and J genes at either the IGL or IGK loci rearrange somatically at the DNA level to generate V-D-J and V-J regions that, after transcription and translation, encode the variable domains of the antibody [4] (for review see [1]).

#### **Key Point 1|Germline.**


#### **Key Point 2|Somatic DNA.**

Somatic DNA is found in all cells of the body (tissues, skin, organs, and blood), except germ cells and embryonic cells, which are the source of gametes. Therefore, a mutation in the somatic DNA is not transmitted to the offspring, but it can lead to the genesis of diseases, especially tumors. So, somatic DNA can be extracted from both tumor (or matched normal) tissue and plasma.
