The 5-HT4 receptor is expressed by virtually all epithelial cells, including EC cells, in the murine colon11. This is a photomicrograph from a section of the distal colon of a 5-HT4R(BAC)-eGFP mouse in which cells that express the 5-HT4 receptor fluoresce green. Green fluorescence is present in all of the epithelial cells, as well as a layer of cells in the muscularis mucosa, and in a myenteric ganglion (block arrow). This section was also processed for 5-HT-immunoreactivity to label EC cells, shown in red (arrows).