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.