In an attempt to characterize connexin genes expressed early in mouse development we screened a cDNA library from mouse F9 embryonic carcinoma cells with mouse connexin37 cDNA and mouse connexin31.1 genomic DNA under low stringency of hybridization. We detected 5 different connexin cDNAs coding for mouse connexins31, -31.1, -32, -43, and -45 (reviewed in Willecke et al., Eur. J. Cell Biol. 56, 1-7 (1991)). Here we describe characterization of mouse connexin31 cDNA coding for a protein of 270 amino acids (Mr 30,905) that shows 8 amino acid exchanges compared to its rat analog recently deduced from its genomic sequence. Mouse connexin45 cDNA codes for a protein of 396 amino acids (Mr 45,671) that exhibits 84% amino acid identity compared to its chick analog described. Cx31 and Cx45 are coded for by single genes in the mouse genome. After Northern blot hybridization, we detected two Cx31 transcripts of 1.9 and 2.3 kb in total mouse RNA from skin, keratinocyte-derived cell lines, and in testis. Cx45 cDNA hybridized to a 2.2 kb mRNA in lung, brain, skin, heart, and intestine. This transcript showed maximal expression in adult lung and in embryonic tissues tested (brain, skin, kidney) where it was at least 40-fold more abundant than in the corresponding adult tissues.