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From a mouse genomic library, a clone has been isolated that codes for a connexin-homologous sequence of 358 amino acids. Because of its theoretical molecular mass of 40.418 kD it is named connexin40 (Cx40). Based on both protein and nucleotide sequence, mouse Cx40 is more closely related to mouse Cx43 (alpha subgroup of connexins) than to mouse Cx32 (beta subgroup). The highest overall homology detected, however, was to chick Cx42 (67% amino acid and 86% nucleotide identity), raising the possibility that Cx40 may be the mouse analogue. The coding region of Cx40 is uninterrupted by introns and is detected as a single copy gene in the mouse genome. High stringency hybridization of Northern blots with the coding sequence of Cx40 identified a single transcript of 3.5 kb that is at least 16-fold more abundant in lung-similar to mouse Cx37-than in other adult tissues (kidney, heart, and skin). In …