Comparison of average expression levels between intronic (n = 14) and intergenic (n = 10) HERV-W elements in eight human tissues. Transcript levels were determined by real-time PCR for each element using assays as described in material and methods and reported as Ct-values. Note that low Ct-values indicate high expression. Bars indicate means and error bars indicate standard deviations (SD). Statistical significance is indicated by ***= p < 0.001.