Fig. 1. Transgene expression and phenotypic outcome. (A) Schematic representation of the wild-type and truncated Dll1 proteins, and structure of the msd::Dll1^dn transgene. In Dll1^dn all but 12 amino acids of the intracellular domain proximal to the transmembrane domain have been removed. For details of the construction of msd::Dll1^dn see Materials and methods. (B) Expression of msd::Dll1^dn in a homozygous day 9.5 transgenic msd::Dll1^dn line 19 (b) and wild-type control (a) embryo visualized by whole-mount in situ hybridisation with an antisense probe specific for the SV40pA sequence. Transgene expression is restricted to the posterior psm and recently formed somites. No expression is detected in the anterior psm corresponding to somitomeres S-1 and S0; psm, presomitic mesoderm. (C) External phenotypes and skeletal preparations of 3-week-old wild-type (a-e) and transgenic (f-v) mice. Dorsal (c,h,o,t) and ventral (d,i,p,u) view of the cervical region, lateral view of the whole vertebral column (b,g,n,s) and thoracic region after removal of the ribs (e,j,q,v). Hemizygous transgenic mice (m-q) show kinky tails (m,n), reduced laminae (black arrow in o), split vertebral bodies (white arrows in p) and reduced or missing pedicles (asterisks in q). Expressivity and penetrance of these defects are significantly increased and also fusions of laminae were observed (arrowheads in h,t) in homozygous animals of independent transgenic lines 13, 19 and 25 (f-l) and in hemizygous msd::Dll1dn line 19 mice that carry only one functional copy of Dll1 (r-v).