Let's look at two separate cases of bone marrow transplantation, one in which the transplant was rejected and one in which the transplant attacked the recipient and caused graft-vs.-host disease.
The donor and recipient were matched by serology but DNA sequencing of the HLA genes showed a one base pair difference between the donor and recipient in one of the HLA molecules.
In both cases the mismatch was caused by one difference, at the same location in the gene that coded for the Class I molecule, B44.
In both cases the single base pair that was different caused amino acid number 156 to be different between the two HLA-B44 molecules (and identified a new B44 allele).