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Occludin has been identified from chick liver as a novel integral membrane protein localizing at tight junctions (Furuse, M., T. Hirase, M. Itoh, A. Nagafuchi, S. Yonemura, Sa. Tsukita, and Sh. Tsukita. 1993. J. Cell Biol. 123:1777-1788). To analyze and modulate the functions of tight junctions, it would be advantageous to know the mammalian homologues of occludin and their genes. Here we describe the nucleotide sequences of full length cDNAs encoding occludin of rat-kangaroo (potoroo), human, mouse, and dog. Rat-kangaroo occludin cDNA was prepared from RNA isolated from PtK2 cell culture, using a mAb against chicken occludin, whereas the others were amplified by polymerase chain reaction based on the sequence found around the human neuronal apoptosis inhibitory protein gene. The amino acid sequences of the three mammalian (human, murine, and canine) occludins were very closely related to each other (approximately 90% identity), whereas they diverged considerably from those of chicken and rat-kangaroo (approximately 50% identity). Implications of these data and novel experimental options in cell biological research are discussed.