Two systems of nomenclature are applied to HLA. The first system is based on serological recognition. In this system, antigens were eventually assigned letters and numbers (eg HLA-B51 or, shortened, B51). Modern HLA nomenclature are begin with HLA- and the locus name, then * and even number of digits specifying the allele. The first two digits specify a group of alleles. Older typing methodologies often could not be completely distinguish alleles and so stopped at this level. The third through four digits specify a synonymous allele. Digits five through six denote any synonymous mutations within the coding frame of the gene. The seventh and eights digits distinguish mutation outside the coding region. Letters such as L,N,Q, or S may follow an allele’s designation to specify an expression level or other non-genomic data known about it. Thus, a completely described allele may be up to nine digits long, not including the HLA-prefix and locus notation (eg, HLA-A*24:02:01 NN=Null) to designate a specific allele at a given HLA locus. Every two years, a nomenclature is put forth to aid researchers in interpreting serotypes to alleles.