The present study applying modern technique and experimental approach where gene expression profiles of MZ twin pairs discordant for MS were compared to each other revealed differential expression of 305 genes out of 8300 genes studied. Among the differentially expressed genes the proportion of up-regulated genes was significantly higher than that of down-regulated ones (64% vs. 36%). This observation may reflect the balance between immunoactivating and immunoinhibitory factors during the complex inflammatory cascade in MS.
It is noteworthy that up-regulated expression of six genes was found in half of twins with MS. This observation together with the fact that the MS twins were obtained from genetically homogeneous Finnish population suggests the importance of these genes in MS. To the best of our knowledge none of the six genes has previously been reported to be associated with MS. The sample was relatively small, but not unexpected given that MS is relatively rare, as is MZ twinning (about 0.4% of births are MZ twin births), The Finnish Twin Cohort covers virtually all twins alive in 1975 and born before 1958 , while the hospital discharge registry identified nearly all MS cases in Finland.
Expression of G1P3 was up-regulated in half of our MS twins. It is known that this gene is transcriptionally induced by IFN-α and -β [21–23], virus infections [24–26] and tumour necrosis factor , but its function remains unknown. It is interesting that Ifi-6-16 peptide translated by G1P3 has been identified as an abundant self-peptide induced following measles virus (MV) infection , which has previously been associated with MS [29–31]. Since most of our MS twins were treated with IFN-β, the up-regulation of G1P3 in their PBMC can most likely be explained by the IFN-β treatment or some unknown virus infection.
Twins with MS had up-regulated expression of POU3F1. The protein (SCIP/Oct-6) translated by this gene has mostly been studied in the nervous system, where it is associated with Schwann cells in the process of remyelination . In oligodendrocytes, cells producing myelin in the CNS, the SCIP/Oct-6 can stimulate the expression of the papovaviral JC regulatory genes in progressive multifocal leukoencephalopathy (PML) , a demyelinating disease of the CNS. However, immune response to the JC virus has not so far been detected in MS.
The MX2 gene was also up-regulated in twins with MS. Mx proteins are induced specifically by IFN-α and -β [34, 35] and have antiviral activities . The antiviral potential or other functions of the MxB protein encoded by MX2 gene are not fully understood, while human MxA protein has a wide antiviral spectrum  and relatively high levels of its mRNA have been detected after treatment with IFN-β . Since up-regulated MX2 gene was observed in MS twins treated with IFN-β, its up-regulation may be explained by this treatment or alternatively by unknown virus infection.
The up-regulated LAPTM5 gene detected in twins with MS is conserved across evolution but it encodes protein which has no homology to any of the other lysosomal proteins .37 LAPTM5 is also known as Clast6 and has been found to be highly expressed in progenitor and precursor B cells . The protein may function during B cell activation or it could have a role in the antigen processing in lysosomes , which may be of relevance in MS.
Up-regulated HBA2 and HBB genes detected in MZ MS twins translate proteins that are part of a hemoglobin molecule. Heme units contain iron, which is involved in myelin production by oligodendrocytes and participates in the initiation of oxidative stress-induced injury in the CNS . It is noteworthy that this process plays a role also in the pathogenesis of MS .