The phenotype observed in DS-epi1 knockout mice is dependent upon the genetic background. Using mice with a pure C57BL6 genetic background, all pups die perinatally, whereas, when using mice with a pure NFR background, approximately half of the pups die. The NFR pups have a retarded growth rate in the late embryological stages of development and, furthermore, ∼ 20% of the pups display gastroschisis, an abdominal wall-closure defect that presents intestines outside the body (R. Gustafsson, unpublished data). DS-epi1 depleted mice in a mixed 129Sv/C57BL6 genetic background have been investigated in more detail. The pups were born at a normal Mendelian frequency 28. At birth, they are smaller and have a crooked tail. Because decorin is a major proteoglycan involved in the organization of collagen fibrils in skin, this tissue was studied in more detail. DS-epi1−/− skin was more fragile than the skin of wild-type mice. DS-epi1−/− collagen fibrils were more heterogeneous in denaturation profiles and in vitro experiments showed that, in DS-epi1−/− skin, decorin was the proteoglycan that was responsible for altered collagen structure (Fig. 4A). Electron microscopy showed
