**3. Gene polymorphisms and haplotypes in the aetiology of multiple sclerosis**

The single-nucleotide polymorphism (SNP) is a variation of a single nucleotide, which is present in the population in a frequency higher than 0.01. SNPs are the most common type of genetic variation and are usually caused by somatic or gametic mutations. Nucleotide change can cause the formation or loss of restriction sites for bacterial endonucleases that are able to cleave the specific DNA sequence. The identification of gene polymorphisms that are in correlation with other risk factors, including biochemical markers, can be useful in establishing the risk of MS development, prognosis, clinical course of the disease and response to therapy. Haplotype (haploid genotype) is a certain combination of alleles or SNPs in the sequence of the DNA, whichis localized on one chromosome and is inherited together. When two alleles are in linkage disequilibrium, they are inherited together in a higher frequency than expected randomly [13]. The combination of more alleles, known as tagging SNPs, enables us to identify the other associated alleles. For example, the allele A of gene polymorphism rs3135388 corresponds to the incidence of allele HLA-DRB1\*1501, which is the most common genetic risk factor for MS development [14–16].
