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A calpain 3 (Capn3) deficiency model was created by targeted disruption of the mouse Capn3 gene through homologous recombination in ES cells. Analysis of the genotype of pups from heterozygous crosses revealed a transmission ratio distortion (TRD) in favor of homozygous Capn3-deficient mice. This TRD was not observed in a second model of Capn3 deficiency, ruling out a possible involvement of Capn3 deficiency in this phenotype. The molecular nature of the TRD was investigated by quantitative RT-PCR and RACE-PCR analyses. We observed the presence in testis and ovaries of abundant, novel transcripts of the Capn3 gene arising from the antisense strand of the Pgk1-neomycin cassette. Although we could not detect corresponding translation products, our results suggest that the activity of the Pgk1 promoter could be the causative factor of TRD. This first example of TRD induced by an introduced cassette further emphasizes the care that should be taken in interpreting phenotypes of animal models, especially when dealing with reproductive functions, and further supports the rationale of using excisable cassettes in inactivation strategies.
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