Dev119529 1193..1202

We have previously described the creation and analysis of a Notch1 activity-trap mouse line, Notch1 intramembrane proteolysis-Cre6MT or N1IP::Cre, that marked cells experiencing relatively high levels of Notch1 activation. Here, we report and characterize a second line with improvedsensitivity (N1IP::Cre) tomark cells experiencing lower levels of Notch1 activation. This improvement was achieved by increasing transcript stability and by restoring the native carboxy terminus of Cre, resulting ina fiveto tenfold increase inCreactivity. Themagnitudeof this effect probably impacts Cre activity in strains with carboxy-terminal Ert2 fusion. These two trap lines and the related line N1IP::Cre form a complementary mapping tool kit to identify changes in Notch1 activation patterns in vivo as the consequence of genetic or pharmaceutical intervention,and illustrate thevariation inNotch1signal strength fromone tissue to the next and across developmental time.