We have previously demonstrated that baculovirus can efficiently transduce human mesenchymal stem cells (MSCs) and MSCs-derived adipogenic, chondrogenic, and osteogenic progenitors without compromising the differentiation capacity. Remarkably, the transgene expression level and duration varied widely with the differentiation states at which the progenitors were transduced. However, whether the variation was a general phenomenon and what caused the variation were unclear. Here we demonstrated that transduction of the MSCs and MSC-derived progenitors using baculoviruses carrying egfp driven by CMV, EF-1 alpha or CAG promoter resulted in a general trend of varied expression, that is, the chondrogenic progenitors allowed for the poorest expression while the adipogenic progenitors conferred the best expression. Quantification of the nuclear and cytoplasmic egfp gene copy numbers by quantitiative real-time PCR revealed that the varied expression did not arise from the discrepancies in gene delivery efficiency nor was it due to the disparities in nuclear transport efficiency. In contrast, the transcription levels paralleled the overall expression levels. These data suggested that although the egfp genes could be efficiently delivered into the nuclei of chondrogenic progenitors, they were not transcribed as well as they were in the adipogenic progenitors. In conclusions, the rapidly altering cellular transcription machinery in the course of differentiation progression predominantly led to the varied expression levels.