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In pigs, changes in an unidentified endometrial glycoprotein, pGP30, are temporally associated with rapid trophoblast elongation and initial placental attachment on day 12 of gestation. Identification of endometrial pGP30 was undertaken through protein purification, NH2-terminal amino acid sequencing and cDNA sequencing of products generated through reverse transcription-polymerasechain reaction. Sequencing of 35 amino acids from the NH2-terminal end of pGP30 revealed that the 30 kDa glycoprotein is a cleavage product from the C-terminal region of inter-α-trypsininhibitor heavy chain 4 (IαIH4), previously known as inter-α-trypsininhibitor heavy chain-like protein. IαIH4 is unique compared with the three other inter-α-trypsininhibitor heavy chains as it does not contain a binding site for bikunin that has serine protease inhibitory activity and is sensitive to cleavage by kallikrein. Endometrial gene expression of IαIH4 was detected during the oestrous cycle (days 0-18)and early pregnancy (days 10-18).Gene expression of IαIH4 appeared to be enhanced during the midluteal phase (days 12 and 15) of the oestrous cycle and the period of trophoblast attachment (days 12-18). Expression of IαIH4 was not detected in day 12 conceptus tissue mRNA. Endometrial expression of IαIH4 in pigs may function as an acute phase protein for protection of the uterus from the inflammatory response induced by conceptus attachment to the uterine epithelium.