Accelerated degradation study: control genes. The bar chart shows the mean total copy number of ABL, BCR, and GUSB control gene transcripts per fixed volume of cDNA for the 4 reference materials after storage at the designated temperatures for 10 months. Frozen baseline: nonlyophilized material from the same batch stored at −150°C. Data at 6 months, storage showed a similar temperature-dependent decline but with less loss of transcript numbers (data not shown).