Abstract:  The colorectal cell line HCA‐7 expresses surface human leucocyte antigen‐A*0201 (HLA‐A*0201), but lacks expression of HLA‐A*0101 whilst the normal B‐cell line (EVA‐1224), derived from the same individual, expresses both surface HLA‐A1 and HLA‐A2. Amplification refractory mutation system‐polymerase chain reaction analysis, using sequence‐specific primers, suggested that HCA‐7 has a mutation in a 7 base pair (bp) cytosine repeat sequence located at the beginning of Exon 4 (bp 621–627). Cloning and sequencing revealed HCA‐7 to have eight cytosine residues in this repeat sequence. In contrast, EVA‐1224 contained only 7 cytosines. Analysis of the mRNA for HLA‐A*010 using reverse trancriptase‐polymerase chain reaction (RT‐PCR), with an allele‐specific 5′ primer in exon 2 (bp 253–271) and a series of 3′ primers in exons 3, 4 and 7 and in the 3′untranslated region, revealed that HCA‐7 contained a shortened message terminating in the region of the exon 3/4 boundary. The insertion of an extra cytosine in this region, which is only two bases from the exon 3/4 splice site, is presumed to lead to a splicing defect between exons 3 and 4 resulting in the lack of expression of a functional HLA‐A*0101 product. HCA‐7 is mismatch repair (MMR) defective due to lack of expression of hMLH1 resulting from hypermethylation of the promoter region. The consequential increase in errors in single‐nucleotide repeat stretches of DNA can account for the HLA‐A*0101 mutation. This has probably then been selected for in the tumour to enable escape from immune attack against an HLA‐A*0101‐restricted tumour‐specific determinant that has also arisen as a result of the tumour being MMR defective.