Integration of the gene-rich metadata from other independent “omics” approaches (DNA/histone chemical modifications, non-coding RNAs, etc.) would definitely TGF-beta enable researchers to come up with a refined genotoxic stress-induced molecular signature that could be used as a biomarker of IR exposure of hESCs. Recently,

the studies in H1 line of hESCs exposed to 1 Gy of IR identified cell growth and proliferation, cell death, DNA-related processes, such as replication, recombination, and DNA repair as being the most genotoxic stress-affected biological pathways/themes[27]. Therefore, it seems that there exists at least partial overlap in major sets of broadly defined processes/functions across distinct hESC lines[23,26,27]. Surprisingly little is known on how low and very low levels of genotoxic stress exposures affect gene expression in hESCs. To the best of our knowledge, our group was the first recently to study the alterations in expression of stress-responsive genes following low and very low doses of IR, such as 0.01 Gy, 0.05 Gy, and 0.1 Gy[28]. The results clearly indicate the heterogeneity of hESCs populations and warrant further genome-wide studies to support the development of “low-dose” specific signature of responses of hESCs. Pluripotent human stem cells are known to present a high degree of heterogeneity in gene expression, but only recently the possible

cause of such diversity was identified by detailed single-cell gene expression studies in hESC subsets defined by surface antigen expression[35]. It was shown that hESC cultures exist as a continuum of intermediate pluripotent cell states[35]. The bulk of the hESC population may express all key pluripotency transcription factors, such as POU5F1, NANOG, SOX2, etc. enabling successful differentiation into derivatives of all three germ layers upon permissive conditions[35,36]. However, a small fraction of hESCs within population shows no lineage priming; these cells possess expression of a particular subset of intercellular signaling molecules with common regulation[35]. Therefore, cultured hESCs can be considered as an inherently Batimastat quasi-stable

population with a multitude of pluripotent states that become committed for lineage specification at some point. The increased expression of developmental regulators in G1 cell cycle might be one of the factors influencing the heterogeneity of hESC populations[37]. The notorious heterogeneity of any stem cell population was recently addressed by single cell quantitative RT-PCR method. It was found that each hESC has high expression in POU5F1, but NANOG expression levels varies significantly[38]. In addition, geometrical position of individual hESCs within each colony can dictate the preponderance to differentiation along specific developmental pathway, such as ectoderm derivatives from the central part of the colony, trophectoderm from the outer colony ring, etc.[39].