The transcription factor OCT4 (officially POU5F1; alternatively OCT3, OCT3/4, OTF3, and OTF4) is currently considered a main regulator of human embryonic stem cell pluripotency and self-renewal capacities. Importantly, these stemness properties are attributed to OCT4A, which is one of the two isoforms produced by the OCT4 gene. The second OCT4 isoform, OCT4B, does not share the stemness factor characteristics of OCT4A and is currently considered of unknown function. Hence, when investigating OCT4 expression at the mRNA and protein level, it is important to specify which OCT4 isoform is detected by the applied methods, such as polymerase chain reaction assays and immunocytochemistry antibodies. Here, we discuss the need to distinguish between OCT4A and OCT4B when interpreting OCT4 expression in differentiated cells, such as peripheral blood mononuclear cells.