ABSTRACT— The cyclin A gene was first identified at a site of hepatitis B virus DNA integration in a primary liver cancer (PLC). It has now been mapped to 4q27, in the proximity of a chromosomal locus (4q32) which is frequently rearranged and deleted in PLC. We took advantage of the TaqI polymorphism recently described in the cyclin A gene to search for allelic loss in this gene by means of PCR. Tumorous and non-tumorous tissue from 50 patients with PLC was analyzed: 27 samples (54%) were homozygous for the A1 type allele (i.e. the allele bearing the TaqI restriction site), three (6%) were homozygous for the A2 type allele (without the TaqI site) and 20 (40%) were heterozygous. Comparison of tumorous and non-tumorous patterns showed that only one (5%) tumorous tissue out of 20 heterozygous patients was homozygous, indicating the loss of an allele at this locus. We conclude that allelic loss in the cyclin A gene is a rare event in patients with PLC, and that PCR is rapid and reliable for the detection of allelic loss in the cyclin A gene when only small amounts of DNA are available.