Objective: OCT4 is a master regulator of self-renewal in embryonicstem cells and can potentially encode two spliced variants, designated OCT4A and OCT4B. Materials and Methods: We have examined the expression pattern of these OCT4 isoforms in various human pluripotent and nonpluripotent cells. Results: Our data revealed that whereas OCT4A expression is restricted to embryonic stem (ES) and embryonal carcinoma (EC) cells, OCT4B can be detected in various nonpluripotent cell types. Furthermore, we detected a novel OCT4 spliced variant, designated OCT4B1, that is expressed primarily in human ES and EC cells and is downregulated following their differentiation. We also found a significantly higher level of OCT4B1 expression in stage-specific embryonic antigen-3 (SSEA3)(+) compared with SSEA3(+) subpopulations of cultured ES cells. Conclusion: Taken together, our data demonstrated a distinctive expression pattern for OCT4 spliced variants in different cell types and highlight the necessity of defining the type of OCT4 when addressing the expression of this gene in different human cells.