Decidual/trophoblast PRL-related protein (d/tPRP) is one member of a large placental PRL gene family composed of at least nine mem- bers in the rat and four in the mouse. Only placental lactogen I and II have been characterized in both rat and mouse. The identification of mouse homologs for rat placental PRL family members will facil- itate gene manipulation studies aimed at identifying functions for these hormones. In this report, we establish the presence of d/tPRP in the mouse and characterize its complementary DNA, protein, and pattern of expression during mouse gestation. Evaluation of the Na- tional Center for Biotechnology Information database of expressed sequence tags resulted in the identification of several mouse comple- mentary DNA clones exhibiting significant homology to rat d/tPRP. One of these clones was obtained from IMAGE Consortium and Re- search Genetics for further investigation. The full-length mouse clone was found to have an 81% nucleotide homology with rat d/tPRP and to encode a 239-amino acid protein. Like rat d/tPRP, the mouse pro- tein contains two putative N-linked glycosylation sites and six ho- mologously located cysteine residues. Mouse d/tPRP maps to chro- mosome 13 along with other members of the mouse PRL family. Like the rat, mouse d/tPRP messenger RNA and protein are expressed by antimesometrial decidual cells and spongiotrophoblast and tropho- blast giant cells in the junctional zone of the placenta. In summary, we have established the presence of d/tPRP in the mouse and dem- onstrated its similarity in structure and pattern of expression to rat d/tPRP. This level of conservation between species expands the bio- logical significance of d/tPRP during pregnancy and provides addi- tional opportunities for evaluating its function. (Endocrinology 138: 5511-5517, 1997)