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A novel member of the insulin superfamily has previously been shown to be expressed only in porcine pre and postnatal Leydig cells and its human analogue demonstrated in the human testes but not in other organs and hence has been tentatively termed Leydig insulin-like peptide (Ley I-L). However, we have detected hLey I-L gene expression in the cyclic human corpus luteum and trophoblast by the reverse transcriptase-polymerase chain reaction (RT-PCR), with primers selected from the published human Ley I-L sequence. Normal and neoplastic breast tissue and fetal membranes with adhering decidua did not express the gene. The overall sequence of the trophoblast gene was in agreement with that reported with minor changes only in the putative connecting peptide, confirmed by restricted enzyme digestion. A 290 bp RT-PCR product was cloned and used as a cDNA probe in Northern analyses; hybridization was readily shown with cyclic corpora lutea but not with other tissues. The broader spectrum of the expression of this gene will warrant a new nomenclature when its biological activities are known. The different intensity of expression in the corpus luteum and trophoblast suggest endocrine and autocrine/paracrine roles respectively in these tissues in which H2 relaxin and H1/H2 relaxins coexist respectively and at similar levels of expression. Operationally the amino acid sequence homologies between the processed H1 and H2 relaxins and hLey I-L may qualify the specificity claimed for immunostaining the human relaxins in the corpus luteum and trophoblast.