Results

The mouse TLS gene was disrupted in embryonic stem (ES) cells. The TLS coding region was interrupted inside exon 8 (immediately upstream of the RNA recognition motif) by a promoterless insertion cassette that creates an allele encoding a truncated TLS–NEO fusion protein that is expressed at low levels (data not shown). This approach was instrumental in ‘trapping’the locus at a very high frequency. Cells derived from the TLS−/− animals express no intact TLS protein (Figure 1A) and the TLS–NEO fusion protein encoded by the targeted allele is expressed at very low levels (supplementary Figure 1; the supplementary data are available in The EMBO Journal Online). Mice heterozygous for the targeted allele are indistinguishable from wild‐type litter‐mates. This finding, together with the very low level of expression of the TLS–NEO fusion protein, implies that the latter does not have discernible neomorphic features. TLS−/− offspring of heterozygote matings are represented at the expected ratio at weaning (96+/+, 200+/−, 85−/−). They are slightly smaller at birth and by the time of weaning, 3 weeks later, mutant animals of both sexes are∼ 70% in size and readily distinguishable from wild‐type or TLS+/− litter‐mates [weight at weaning of TLS−/− males 9±2 g versus wild type 15±2 g (n= 25); TLS−/− females 8±2 g versus wild type 14±2 g (n= 25)]. Other than their reduced size, the mutant animals appear developmentally normal. In a specific‐pathogen‐free animal facility, the survival of TLS−/− animals of partially outbred background (with equal contribution of genes from the 129svev and CD1 strains) is virtually unimpaired. In the inbred, 129svev background, rare mutant animals are alive at weaning, but none reach adulthood. The cause of this perinatal attrition of TLS−/− animals in the inbred background is not known.