One gap junction consists of a cluster of closely packed pairs of transmembrane channels, the connexons, through which materials of low MW diffuse from one cell to a neighboring cell. Involved in tracer coupling between horizontal cells of the retina. May play a role in the regulation of horizontal cell patterning (By similarity). A connexon is composed of a hexamer of connexins. Expressed in skeletal muscle and heart. Belongs to the connexin family. Alpha-type (group II) subfamily.