The protein structural component of gap junctions is the connexin. Studies on the association properties of the connexins to form heteromeric connexons and heterotypic gap junctions are necessary for a complete understanding of the role of different connexins in gap junction function. The connexins are coded by a multigene family consisting of at least 16 members. Most cells express multiple types of connexin that can potentially associate to form gap junction channels containing more than one type of connexin. The permeability and gating characteristics of gap junction channels are dependent on the isoform and post-translational modifications present on the connexins and their association properties. Together with an observed selectivity in the association properties of the different connexins and the development of more specific perturbation approaches, these studies have provided insights into the significance of connexin diversity and the temporal expression patterns for connexins that have been determined in vivo in both developmental and differentiating systems.