To provide novel insights into the molecular events associated with parturition, a differential screen was made of a mouse uterine complementary deoxyribonucleic acid library to identify selectively expressed genes in late pregnancy.A differential hybridization was used to screen a mouse complementary deoxyribonucleic acid library prepared from late pregnancy uterus. A 1 kb clone was isolated that was subsequently identified as 24p3, a member of the lipocalin family. By use of radiolabeled complementary deoxyribonucleic acid probes prepared from this clone Northern hybridizations were conducted against total ribonucleic acid purified from mouse uterus collected during late pregnancy and the first week after birth and a variety of other mouse tissues to determine whether this gene is selectively expressed in the uterus coincident with parturition.Low levels of message for the 24p3 gene could be detected in uterine ribonucleic acid, but there was a massive increase in the level of message for this gene on the days surrounding birth. Northern hybridizations conducted against additional tissues collected from both pregnant and nonpregnant mice did not detect message to a similar degree as found in the uterus.The uterus constitutes a major site of expression of this gene, particularly near birth. The expression of this gene coincident with birth suggests a potential physiologic role of the neutrophil with the induction of labor.