Gene trapping with the retroviral ROSA beta geo vector was used to generate lines of mice carrying disrupted genes. Both cDNA and genomic flanks have been cloned from a number of these lines. One mutation has been shown to disrupt the alpha-enolase gene by insertion of the splice-trap vector into the first intron. In adult mice, lacZ expression was detected only in testes. Embryonic expression was detected from 10.5-day postcoitum embryos and was seen as a diffuse staining pattern over much of the embryo, consistent with the housekeeping gene function of alpha-enolase. This mutation results in an early recessive embryonic lethality. Mice heterozygous for the mutation have no obvious phenotype. Mutations of this gene in humans are reported to be associated with rare autosomal-dominant, non-spherocytic haemolytic anaemia. This phenotype is not reproduced in mice heterozygous for this mutation.