The mRNA expression of the ESX1L gene was analyzed by RT-PCR and in situ hybridization in human normal cytogenetically placentas, of different gestational ages. Our RT-PCR analysis showed that ESX1L mRNA is expressed from 5 weeks of gestation until term, suggesting a role not only in trophoblast differentiation but also in the maintenance of the villi and microvasculature. We also observed, by in situ hybridization, that ESX1L mRNA is expressed by cytotrophoblast from chorionic plate, syncytiotrophoblast and stromal cells of all terminal, intermediate and stem villi of term placentas. ESX1L mRNA expression was more pronounced in trophoblast cells of terminal villi than in intermediate and stem villi. In conclusion, ESX1L is expressed during all stages of placental development and is localized to sparse areas of trophoblast in terminal villi in association with cytotrophoblastic cells.