Exon 2 nucleotide sequence of the DRB1 gene encoding the HLA-DRw13b allele defined by DNA-RFLP (Restriction Fragment Length Polymorphism) typing, has been obtained by using five heterozygous individuals genomic DNA and a non isotopic automated "dideoxi" methodology. Its comparison with other known homologous DRB1 sequences suggests that two different mechanisms which generate HLA allele variability may have occurred in this particular exon 2: a gene conversion between DRw11 or DRw13 as acceptors and DR4-Dw15 or DRw8.1 as donors and in addition, a non-conservative point mutation at base 221. The relationship between this HLA sequence characteristics and certain diseases susceptibility is discussed.