The PDZ domain gained its name from the three proteins that were first seen to have homology by virtue of these domains, the mammalian postsynaptic density protein, PSD-95, the Drosophila discs-large septate junction protein, DLG, and the mammalian epithelial tight-junction protein zona occludens, ZO-1. Over 50 PDZ domain-containing genes have been recognized so far from almost any organism subjected to sequencing, including mammals, nematodes, yeast, plants, and bacteria. The domain consists of an approximately 90-amino-acid-residue unit, which is often repeated in the protein. The majority of residues form a conserved spatial structure while a few amino acids in critical positions confer protein binding specificity. A subgroup of PDZ domains have been shown to recognize a short carboxy-terminal amino acid motif, T/SXV (Ser/Thr-X-Val-COO-), where X is any amino acid. We have identified and completely sequenced a gene, Mpdz, that encodes a mouse protein containing 13 such domains. We have also mapped the gene to a series of overlapping deletions on mouse chromosome 4 and can therefore determine that its function is not essential for embryonic development or neonatal survival.