Example of a spectrum observed by IRIS in the AR 12356 (see Sect. 4.2) showing emission lines from the transition region. The figure shows the O iv + S iv blend at around 1404.82 Å, and the wings of the O iv 1401.16 Å and S iv 1406.06 Å lines blending with photospheric cooler transitions (S i and Fe ii respectively). In addition, we note that the shape of the IRIS line profiles for these ions is not Gaussian, but rather similar to a generalized Lorentzian function.