Temporal expression pattern of PrP during Xenopus development. Total RNA was extracted from embryos at the indicated developmental stages and used for RT-PCR analysis (staging according to Nieuwkoop and Faber (1967)). Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used as a control for the amount of input RNA. The generated PCR products were 225- and 230-bp in size for PrP and GAPDH, respectively. Negative controls without reverse transcriptase (not shown) or without cDNA for each primer pair used (H2O) showed no contamination of genomic DNA for all developmental stages.