TWIK-1; Expression of TWIK-1 was detected in numerous cell types of the adult mouse retina (Fig. 2), and showed a different pattern of expression throughout postnatal development (Supplementary Figure 2). In the adult retina, TWIK-1 immunoreactivity was detected predominantly in Mϋller cells and a subset of cells located in the ganglion cell layer (GCL) (Fig. 2A–C). The intensity of TWIK-1 immunoreactivity within Müller cells was found to be somewhat variable across the adult retina, with areas of high and low expression often observed in the same retina sections. Based on our analysis it was not possible to determine the extent or patterning of this distribution across the entire retina. Double labelling for the retinal ganglion cell marker Brn3a (brain-specific homeobox/POU domain protein 3A) that is expressed in the majority, but not all classes of retinal ganglion cells61, confirms the expression of TWIK-1 within 30.7% of RGCs (68 of 196 cells counted), but was also detected in a number of Brn3a negative cells (Fig. 2D–F). In addition to expression of TWIK-1 in Mϋller cells and RGCs of the adult retina, we also observed TWIK-1 immunoreactivity during early development in a population of cells located in the middle of the neuroblastic cell layer, resembling developing horizontal cells (Supplementary Figure 2). Immunoreactivity was weak in these cells at P3, and was increased by P5. By P10 the morphology of these cells clearly resembled horizontal cells, with strong labelling of processes evident in the forming outer plexiform layer. TWIK-1 immunoreactivity was absent from these presumed horizontal cells after P10. Although expression of the RGC marker Brn3a was detected as early as P0, TWIK-1 immunoreactivity was absent from these developing ganglion cells prior to P10 and shows a significant increase by P14 and P30 (Supplementary Figure 2). TWIK-1 immunoreactivity was absent from Müller cells during early postnatal development (Supplementary Figure 2).