In the present study, we performed microarray analysis during BM-MSC chondrogenesis in vitro. We found that over 1,486 genes were expressed by BM-MSCs during chondrogenesis, and we identified genes that were differentially expressed. These data may provide novel information of the genes involved in chondrogenesis of human BM-MSCs. Identification of CD8(+ )cytotoxic T lymphocyte epitopes from porcine reproductive and respiratory syndrome virus matrix protein in BALB/c mice Twenty-seven nanopeptides derived from the matrix (M) protein of porcine reproductive and respiratory syndrome virus (PRRSV) were screened for their ability to elicit a recall interferon-γ (IFN-γ) response from the splenocytes of BALB/c mice following DNA vaccination and a booster vaccination with recombinant vaccinia virus rWR-PRRSV-M. We identified two peptides (amino acid residues K(93)FITSRCRL and F(57)GYMTFVHF) as CD8(+ )cytotoxic T lymphocyte (CTL) epitopes. These peptides elicited significant numbers of IFN-γ secreting cells, compared with other M nonapeptides and one irrelevant nonapeptide. Bioinformatics analysis showed that the former is an H-2K(d)-restricted CTL epitope, and the latter is an H-2D(d)-restricted CTL epitope. Multiple amino acid sequence alignment among different PRRSV M sequences submitted to GenBank indicated that these two CTL epitopes are strongly conserved, and they should therefore be considered for further research on the mechanisms of cellular immune responses to PRRSV. Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important swine viral pathogens, and has caused significant economic losses to the swine industry worldwide. Characterization of field isolates suggested that PRRSV are genetically diverse, and this genetic variation increases the difficulty of developing effective vaccines. Based on significant sequence differencesPRRS viruses are grouped into two distinct genotypes, European isolate (Lelystad virus, LV) and North American isolate (VR-2332) [1] PRRSV has two major structural proteins, GP5 and M, encoded by ORFs 5 and 6, respectively. GP5, the most important neutralizing antigen of PRRSV, has the highest genetic diversity among isolates [2] . And, recent studies in Yorkshire × Landrace crossed and outbred pigs, showed that there are two immuno-dominant T-cell epitopes derived from the GP5 protein: L 117 AALICFVIRLAKNC and K 149 GRLYRWRSPVII/VEK [3] . The M protein, which contains highly conserved amino acid sequences, also has very good immunogenicity and is associated with protection against PRRSV infection. DNA vaccinations have also revealed that M is the most potent inducer of T lymphocyte proliferation [4] .