This mutation, a deletion in chromosome 1 between bp 260,070,892 and 260,166,363, was originally found in the offspring of Hsd:SD x Hsd:SD. The phenotype was an altered titin isoform expression in the offspring. The parent carrier was identified by crossing both the male and the female Hsd:SD with F344/NHsd. This mutation is maintained on Hsd:SD X F344/NHsd.