Since the introduction of DNA-based human leukocyte antigen (HLA) typing a number of discrepancies with serological typing have been documented. At the time of submission of this abstract (July 2005 ImMunoGeneTics project (IMGT) project database release) 42 HLA class I and II null alleles had been described characterised by a lack of expression of cell surface antigen. These null alleles can be accounted for by a number of demonstrated molecular mechanisms including insertion, deletion and point mutation and may lead to a nonsense codon, splicing defect or premature stop codon.
Fidler, S. J., Smith, L. K., & Christiansen, F. (2007). The identification and characterisation of a novel null HLA-DRB1 allele. Tissue Antigens, 69(Supplement 1), 58-60. https://doi.org/10.1111/j.1399-0039.2006.760_3.x