Both Specific Aims of this Seed Grant were accomplished. First the intermediate filament composition of hES cells was compared to mouse ES cells and was found to more resemble that of mouse epiblast cells than mouse inner cell mass. Thus hES cells are shown to express keratin 8, 18 and 19, like mouse epiblast and unlike mouse ES cells. Second the early expression of keratin 7 was correlated with differentiation of hES cells to trophoblast derivatives. Gene expression alterations associated with the treatment of human ES cells with BMP4 confirmed the activation of many genes previous implicated in mouse trophoblast stem cells. A Cdx2GFP reporter gene was introduced into the BG01v hES cell lines and found to express GFP upon differentiation to the trophoblast lineage. However, this line still needs further characterization. Additional testing of the reporter gene was terminated because of the end of funding.