Connexins are associated with which of the following?

At gap junctions, the intercellular space narrows from 25 nm to 3 nm, and units called connexons in the membrane of each cell are lined up with one another. Each connexon is made up of six protein subunits called connexins. They surround a channel that, when lined up with the channel in the corresponding connexon in the adjacent cell, permits substances to pass between the cells without entering the ECF. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 2.