Basic principles of RQ-PCR–based MRD analysis using rearranged IG and TR genes as targets. (A) Schematic diagram of an IGH gene rearrangement, resulting in a V-D-J exon with highly diverse junctional regions, which differ in each individual B cell, even if by coincidence the same V, D, and J genes are used. (B) RQ-PCR analysis of an dilution experiment, showing the technical definitions for interpretation of RQ-PCR results.¹⁸  The amplification plot shows the position of the threshold and obtained Ct values, the quantitative range, the sensitivity, and the background signal. (C) Example of RQ-PCR MRD analysis using an Vδ2-Dδ2-Jα11 rearrangement as target.⁴⁴  One primer and the TaqMan probe are positioned at the Vδ2 gene and the other primer is an ASO primer, positioned at the Vδ2-Dδ2 junctional region. The amplification plot (right) shows the dilution experiment and the follow-up sample (in triplicate). The corresponding standard curve (left) is based on the dilution experiment and allows calculation of the ALL cell frequency in the follow-up sample.