Both RPP30 synthetic construct and human genomic DNA NA10860 from the Coriell Cell Repositories (Camden, NJ, USA) were quantitated using the RPP30 assay on a digital array. Different amounts of RPP30 synthetic construct were then spiked into the genomic DNA so that mixtures with ratios of RPP30 and RNase P of 1 : 1 (no spike-in), 1 : 1.5, 1 : 2, 1 : 2.5, 1 : 3 and 1 : 3.5 were made, simulating DNA samples containing two to seven copies of the RPP30 gene.