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The doubleridge mutant was identified in a phenotype screen of a set of transgenic mouse lines for recessive insertional mutants. The mutation resulted from random insertion of a 50-kb tandem transgene cluster accompanied by deletion of a 60 kb DNA segment between and downstream (3') of two oppositely transcribed genes: Mbl2 and Dkk1, at respective distances of 90 kb and 150 kb. The insertion/deletion occurred in a chromosomal segment derived from the SJL parent of the microinjected hybrid zygote. The transgene is not expressed. In heterozygous mutant mice, significantly lower levels of Dkk1 mRNA are expressed from the chromosome bearing the mutation than from the wild-type chromosome. Transcript levels do not differ between mutant and non-mutant chromosomes for Mbl2 or Prkg1, which lies upstream of Mbl2. (J:91047)