Recent genomic evidences from unfractionated embryonic stem cell (ESC) cultures have demonstrated high levels of concomitant activating (H3K4me3) and repressive (H3K27me3) histone methylations, termed “bivalent marks”, at lineage specific gene loci, demonstrating that all cells residing within the cultures are developmentally equipotent. However, this dogma has been challenged, indicating that ESC cultures are heterogeneous, with individual cells displaying dynamic metastability and failed to make a connection with the variations between cell lines, a broad spectrum of differentiation, continuous phenotypic oscillation, and the expression of lineage specific genes in undifferentiated state. Recently, functional in vitro assays via fractionation of ESC cultures based on comparable expression of some phenotypes (c‐KIT, A2B5, SSEA3, Nanog, Rex‐1, IGFR1, and Stella) revealed a plastic gradient of clonogenicity and lineage specification within ESC cultures reflected by the presence of bivalent marks, which are resolved down to activating “monovalent marks”. More interestingly, dynamic heterogeneity represents a conserved feature on both mouse ESCs and human ESCs as being essentially required for self‐renewal and, more importantly, differentiation. However, it is the most substantive obstacle to control and specify ESCs into desirable cell types. Mostly, differentiation from ESCs has been evaluated by measuring the responses of whole EB populations under the specific inducible conditions, making it difficult to identify, which cell populations are dominantly contributing to differentiated progeny from ESCs. Therefore, further identification of novel transcriptional and phenotypic markers may allow for the isolation and enrichment of more promising target cells for stem cell‐based clinical therapy.

• Seok-Ho Hong(Stem Cell Laboratory, CReATe Program, Sunnybrook Women's College Hospital, Canada)