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. A connexon is composed of a hexamer of connexins. Interacts with CNST. Highly expressed in adult brain and skin. Less in uterus, lung and eye. Very low in testis and sciatic nerve. No expression before birth. Belongs to the connexin family. Beta-type (group I) subfamily.