The connexin genes Cx31 and Cx45 coding for proteins of gap junctional subunits have been assigned to mouse chromosomes 4 and 11 by Southern blot hybridization of specific gene probes to DNA from mouse x Chinese hamster somatic cell hybrids. In addition, our results confirm the recent assignment of mouse connexin genes Cx26, Cx32, Cx37, Cx40, Cx43, and Cx46 to mouse chromosomes 14, X, 4, 3, 10, and 14, respectively, by analysis of interspecific backcrosses and by somatic cell hybridization. Our assignment of the Cx31 gene to mouse chromosome 4 locates the fourth connexin gene on this mouse chromosome to which the genes for Cx31.1, Cx37, and Cx30.3 have previously been assigned. Interestingly three of them (coding for Cx31, Cx31.1, and Cx30.3) are preferentially expressed in skin. Possibly some of the connexin genes clustered on mouse chromosome 4 may be regulated coordinately.