Individual donor variation in siRNA down regulation for three individual donors examined. Quantitative PCR analysis data for transfection with (a) UGT1A9 or (b) UGT2B7 siRNA, indicates selective down regulation of the intended target gene, with minimal variation among hepatocyte donors (Hu4199, Hu4237, and Hu4151). An asterisk (*) indicates a statistically significant difference, where p < 0.001. Error bars indicate the standard deviation for replicate incubations (n = 3).