A sample typed by SSP in 1996 was retyped in 1998 with different results. What is the best explanation for this discrepancy?

The best explanation for the discrepancy in typing results from samples typed in 1996 and retyped in 1998 using Sequence-Specific Primers (SSP) lies in the occurrence of new alleles that may exhibit the same reactivity patterns as previously characterized alleles. Over time, advancements in genetic research and expanded databases of known alleles can lead to the identification of new variants. These new alleles, while distinct at the sequence level, could potentially react in a manner similar to older alleles when subjected to the same primer sets.

This phenomenon illustrates the evolving nature of human leukocyte antigen (HLA) typing, where the introduction of new alleles may confound previous typing results if the SSP used were designed based on older allele knowledge. Since allele characterization and understanding are continually updated as additional research is conducted and databases expand, it is possible for a re-evaluation or retyping using the same methods to yield different results due to the identification of homozygous or new alleles sharing similar primer binding affinities.

Other explanations, while interesting, do not satisfactorily account for the specific issue of typing discrepancies due to the introduction of new alleles that may share similar primer reactivity.