HI spectra of four isolated UDGs, obtained with the Effelsberg radio telescope. From top to bottom, the panels correspond to DGSAT I, M-161-1, R-127-1, and SdI-2. The first three spectra are non-detections. SdI-2 is instead clearly detected in the last spectrum. The light blue solid line is the best fitting generalized busy function profile (Westmeier et al. 2014). In all spectra, the short vertical lines denote the expected systemic velocity range from prior optical redshift measurements.