Left panel: continuum-subtracted rms spectrum of APM 08279+5255 Si IV emission line from our RM monitoring. Right panel: continuum-subtracted HST spectrum of the C IV emission line. In both panels, errors on the flux level (grey bars) and best-fit Gaussians (red curves) are shown. For Si IV, the major deviations from the best-fit curve are the residuals of intrinsic narrow absorptions superimposed to the emission line, as can be seen from the analysis by Srianand & Petitjean (2000) and Ellison et al. (2004).