PrP levels in prion-infected brains of C57BL/6 and <i>Tg</i>a<i>20</i>, and specific infectivities of PK-resistant PrP.

The PrP content of brain homogenates was assessed by western blot and quantification of immunoreactive bands in arbitrary pixel units by UVP (BioSpectrum®AC Imaging System). Each measurement was adjusted for gel loading to provide the pixel value per µg of sample protein. All gels have been normalized to each other allowing the comparison of PrP values. TL- and PK-resistant PrP values have been calculated using regression curves of signal intensities obtained on serial dilutions of each sample (Fig. 2 B and C and Fig. S3). *The specific infectivity of resPrPSc is herein defined as the ratio of the RI measured in LD9 cells (susceptible to all three prion strains) relative to resPrPSc (pixels per ng of brain protein, see above).