One V and one J gene are joined --> random number of nucleotides are added to or deleted from the ends of the V and J genes.

Resulting junction is referred to as complementarity determining region 3 (CDR3).

CDR3 encodes the site of antigen binding which determines a lymphocyte's specificity.

Clonality Testing helps to determine whether a population of cells is derived from a single parental clone, or if that population is heterogeneous.

PCR for antigen receptor gene rearrangement (PARR) is a method for detection of clonal lymphocyte populations based on assessment of diversity within the CDR3.

T cell receptor gamma (TCRG) genes --> T-cell lymphoma

Immunoglobulin heavy chain genes (IGH) --> B-cell lymphoma

The term PARR is no longer used in veterinary medicine and was changed to Clonality Testing as in human medicine.