Unprocessed pro-Neuregulin 1 (type I) accumulates as discrete puncta on the soma and proximal dendrites of cultured hippocampal neurons at contact sites, known as subsurface cisterns, between the somatic plasma membrane and the ER (white). Note that Neuregulin puncta are absent from axons (initial segments labeled with Ankyrin G, green) and more distal dendrites (labeled with MAP2, magenta). In response to NMDAR activity pro-NRG1 is processed and released. For more information, see the article by Vullhorst, Ahmad et al. (pages 5232–5249).