Light-chain variable region gene, protein, and PCR.

(A) The upper sketch shows the Leader,V_L (variable), J_L(joining), and C_L (constant) gene segments as found in an expressed gene, and the various sets of primers that may be designed to amplify the variable region gene or portions of it. The lower sketch shows a schema of the light-chain protein and its regions. Framework regions (FR1-4) provide structure and usually have few amino acid replacements due to somatic mutation. Complementarity-determining regions (CDR1-3) provide sites for antigen binding and have frequent amino acid mutations due to somatic mutation. The constant region (C) also provides structure. Note that theVJ junction comprises the middle of the CDR3 region. (B) A PCR gel is shown of the same V_λVI light-chain cDNA amplified with different sets of primers. Lanes 1 through 4 show segments of different length amplified with primers to different portions of the gene: lane 1, L-3′ C_Lprimers → 708 bp; lane 2, L-5′ C_Lprimers → 435 bp; lane 3, FR1-5′ C_L primers → 378 bp; and lane 4, CDR1-CDR3 primers → 231 bp. PCR with unique CDR1-CDR3 primers is useful for detecting minimal residual disease and clonotypic contamination of stem cell components. M_X indicates size markers as shown in legend; B, no cDNA blank; bp, base pairs.