All experiments were performed in triplicates. Chromatin from MCF-7 cells (1 million per sample) were subjected to ChIP assay with anti-RNA Polymerase II (Pol 2) antibody followed by real-time PCR analysis of the proximal promoter of GAPDH, and an ORF-free region (IGX1A). Users A and B performed the PCR assays either in 96-well or 384-well plate format, on a Stratagene Mx3005 or an ABI 7900 real-time PCR instrument, respectively. The same ChIP DNA samples were used after storage for extended periods of time as indicated. The results demonstrate high reproducibility of PCR performance across technical replicates, lots, instruments, and differential handling.