This image shows reactive Müller glia in the mouse retina. The Müller glia are stained with a cell-type specific marker, glutamine synthetase (green) and a marker of glial reactivity, glial fibrillary acidic protein (red). Loss of Mertk function in Müller glia is sufficient to cause activation of Müller glia that leads to impaired synaptic development in the retina. For more information, see the article by Koh et al. (pages 2923–2943).