GO:0005921

A cell-cell junction that is composed of an array of small channels that permit small molecules to pass from one cell to another. At gap junctions, the membranes of two adjacent cells are separated by a uniform narrow gap of about 2-4 nm that is spanned by channel-forming proteins called connexins, which form hexagonal tubes called connexons. [GOC:mah, GOC:mtg_muscle, http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/junctions_g.html, ISBN:0815332181]