A nucleotide insertion in exon 4 is responsible for the absence of expression of an HLA-A*01 allele.

HLA class I typing performed in parallel by molecular biology and serology has revealed cases where an HLA class I allele was identified whereas the corresponding antigen was not detected on the cell surface. In the present report, we describe four members of a family in whom an HLA-A1 allele identified at the molecular level was typed as A "blank" by lymphocytotoxicity. This serologically blank antigen was undetectable by isoelectric focusing (IEF). Sequencing of the HLA-A01 allele from the promoter region to the eighth exonic region revealed insertion of a "C" nucleotide at the beginning of the fourth exon as compared to the common HLA-A0101 allele. This mutation causes a frame shift, giving rise to an early stop codon in the fourth exon.