Indeed, our results suggest that once these culture-induced factors are eliminated, hESC derived from different stages of embryo development exhibit very little difference in their gene expression profiles and maintain similar pluripotent phenotype. One explanation for this finding is that although the source cells from which hESC are derived are clearly at different developmental stages, the process of hESC derivation selects for cells that assume certain phenotypic and molecular state required for the survival and self-renewal of pluripotent cells in culture. One of the unique aspects of our analysis is that unlike the cell lines generally used for gene expression studies that were often derived and grown under heterogeneous culturing conditions and have typically undergone extensive passaging and selection in culture, lines used in this analysis were derived and grown under identical defined conditions for limited number of passages, thus minimizing culture-induced changes. While overall transcriptional profile did not appear to be significantly altered between blastomere and ICM-derived hESC, we did detect a small number of genes with significantly different expression between two groups. Out of these, only CLC, ZNF558 and LGALS14 genes were down-regulated more than 4 fold in blastomere derived hESC lines. Interestingly, two of the genes encode proteins from galectin family characterized by presence of carbohydrate ICI 182780 129453-61-8 recognition domains and bgalactosidase binding activity. CLC whose expression is downregulated 20 fold, encodes a lysophospholipase, an enzyme that hydrolyzes lysophosphatidylcholine to glycerophosphocholine and a free fatty acid and thus, contributes to the regulation of cell membrane function. LGALS14 gene is predominantly expressed in the placenta, an extraembryonic tissue that develops after blastocyst implantation. The significance of their low abundance in blastomere lines remains to be elucidated. Our analysis does not preclude the possibility that there are additional subtle differences in molecular and phenotypic profiles between cleavage stage blastomere-derived and blastocyst stage ICM-derived hESC lines. For example, epigenetic changes including differences in DNA methylation patterns and X chromosome inactivation have been shown to be affected by different hESC states. In addition, while we have not observed differences in the ability of blastomere-derived and ICMderived hESC lines during spontaneous differentiation in vitro and in vivo to give rise to cells from all three germ layers, efficiency of differentiation and differentiation propensity towards specific cell lineages have not been evaluated and are subject of current investigation in our laboratory. Nevertheless, overall our data strongly indicate that blastomerederived lines exhibit all the key characteristics of hESC derived from ICM, including presence of pluripotency-associated pathways, ability to differentiate into cells from all three germ layers, self-renewal and hESC-specific transcriptional profile. This suggests that they represent a valid, embryo friendly.