Bloom syndrome (BS) is an autosomal recessive disorder in which mutation of the BLM gene leads to cancer predisposition. In the September 20 Science, two papers suggest that BLM haploinsufficiency is sufficient for enhanced tumorigenesis. Kathleen Goss and colleagues describe the generation of a mouse model for BS by creating a Blm^Cin mutation that mimics the mutated BLM^Ash allele found in Ashkenazi Jewish patients (Science, 297:2051-2053, September 20, 2002). Heterozygote Blm^Cin/+mice died earlier than controls following infection with murine leukemia virus and showed increased numbers of intestinal tumours when crossed with animals carrying the Apc^Min mutation. The analysis by Goss et al. suggests that mutation of the wild-type Blm allele was not required for tumour formation.