These mice contain a sequence encoding an intracellular portion of the mouse Notch1 gene (amino acids 1749-2293), but lacking the c-terminal PEST domain, and Green Fluorescent Protein, GFP, inserted into the GT(ROSA)26Sor locus. Expression of the Notch1 fragment and GFP is blocked by a loxP-flanked STOP fragment placed between the coding sequence and the GT(ROSA)26Sor promoter. The GFP expression is localized to the nucleus by an IRES sequence. The truncated cytoplasmic fragment encoded by the Notch1 sequence causes constitutive signaling activity. When used in conjunction with a Cre recombinase-expressing strain, this strain is useful in generating tissue-specific mutants for studying the effects of Notch pathway activation. Homozygous mutant mice are viable, fertile, normal in size and do not display any gross physical or behavioral abnormalities. For example, when crossed to a strain expressing a tamoxifen inducible Cre recombinase in all cells that express Shh (see Stock No. 005623), this mutant mouse strain may be useful in studies of Notch signaling. When crossed to a strain expressing a tamoxifen inducible Cre recombinase in all cells that express Neurog3 such as spermatogonia and pancreatic islets cells (see Stock No. 008119), this mutant mouse strain may be useful in studies of Notch signaling. When bred to a strain expression interferon inducible Cre recombinase in liver and lymphocytes (see Stock No. 003556 for example), this mutant mouse strain may be useful in studies of Notch signaling in lymphocyte development. The Jackson Laboratory 2024, used with permission.